The existing Detector interface cannot be extended in a backward
compatible way to support new features desired for the GitDetector
implementation[0]. The new CtxDetector interface is introduced to allow
such extension without breaking backward compatibility with existing
users.

The new interface also avoids adding new methods to the existing
Detector interface that would then need to be supported by all Detector
implementations, both in-library and in the wild.

A CtxDetector is slightly more cumbersome to use than Detector. Callers
can (and should) continue to use Detector unless the enhanced
capabilities of one or more of the CtxDetector implementation modules is
needed. At the time of writing (2020-08), the only CtxDetector with such
extra mojo is the forthcoming GitCtxDetector.

Existing Detector implementations can easily be wrapped by CtxDetector
implementations. The information available to a CtxDetector impl. is a
strict superset of the information provided to a Detector. Where there
is no need for the additional context info provided by the CtxDetect
dispatch function, impls. can simply pass through the common subset to
the Detect method of the analogous Detector impl.

CAVEAT: In this changeset the list of ContextualDetectors is commented-
        out. This is intended to make a clear introduction of the
        interface type prior to introducing any implementations of it.

A forthcoming change will provide such wrapping for all in-tree Detector
impls, followed by the introduction of specialization for the
GitCtxDetector impl.

[0] C.f., hashicorp#268

The go-getters library now has support for local file system paths to
Git repositories, specified with the 'git::' forcing token. The feature
works for both absolute and relative filepaths, and supports all the
usual go-getter goodies including '//' delimited subdirs and URI-style
query parameters.[0][1]

We incorporate that capability into Terraform, which allows users to
specify paths to locally present Git repositories from which to clone
other Terrform modules on which they are dependent. When coupled with
Git submodules, this creates a powerful way to manage Terraform modules
at specific versions without requiring those modules to be available on
the network (e.g., on GitHub):

    module "my_module" {
        source = "git::../git-submodules/tf-modules/some-tf-module?ref=v0.1.0"
        // ...
    }

From the perspective of Terraform, such Git repositories are "remote" in
the same way that repositories on GitHub are.

Note that within a Terraform module "call" block, the filepaths
specified are relative to the directory in which the *.tf file lives,
not relative to the current working directory of the Terraform
process. In order to support this feature, Terraform needs to supply
that contextual information to go-getter to allow relative filepath
resolution to work. In order to do so, we needed to switch over to using
go-getter's new "Contextual Detector" API. It works in the same basic
way as the traditional Detector API, but allows us to provide this
additional information.

In keeping with the "keep things simple" comment in the commit message
of 2b2ac1f, we are here maintaining our custom go-getter detectors
in two places. Only now each is called FooCtxDetector rather than
FooDetector. Nevertheless, all except the GitCtxDetector do little more
than "pass through" delegation to its analogous FooDetector counterpart.

Fixes hashicorp#25488
Fixes hashicorp#21107

[0] hashicorp/go-getter#268
[1] hashicorp/go-getter#269

The existing Detector interface cannot be extended in a backward
compatible way to support new features desired for the GitDetector
implementation[0]. The new CtxDetector interface is introduced to allow
such extension without breaking backward compatibility with existing
users.

The new interface also avoids adding new methods to the existing
Detector interface that would then need to be supported by all Detector
implementations, both in-library and in the wild.

A CtxDetector is slightly more cumbersome to use than Detector. Callers
can (and should) continue to use Detector unless the enhanced
capabilities of one or more of the CtxDetector implementation modules is
needed. At the time of writing (2020-08), the only CtxDetector with such
extra mojo is the forthcoming GitCtxDetector.

Existing Detector implementations can easily be wrapped by CtxDetector
implementations. The information available to a CtxDetector impl. is a
strict superset of the information provided to a Detector. Where there
is no need for the additional context info provided by the CtxDetect
dispatch function, impls. can simply pass through the common subset to
the Detect method of the analogous Detector impl.

CAVEAT: In this changeset the list of ContextualDetectors is commented-
        out. This is intended to make a clear introduction of the
        interface type prior to introducing any implementations of it.

A forthcoming change will provide such wrapping for all in-tree Detector
impls, followed by the introduction of specialization for the
GitCtxDetector impl.

[0] C.f., hashicorp#268

The existing Detector interface cannot be extended in a backward
compatible way to support new features desired for the GitDetector
implementation[0]. The new CtxDetector interface is introduced to allow
such extension without breaking backward compatibility with existing
users.

The new interface also avoids adding new methods to the existing
Detector interface that would then need to be supported by all Detector
implementations, both in-library and in the wild.

A CtxDetector is slightly more cumbersome to use than Detector. Callers
can (and should) continue to use Detector unless the enhanced
capabilities of one or more of the CtxDetector implementation modules is
needed. At the time of writing (2020-08), the only CtxDetector with such
extra mojo is the forthcoming GitCtxDetector.

Existing Detector implementations can easily be wrapped by CtxDetector
implementations. The information available to a CtxDetector impl. is a
strict superset of the information provided to a Detector. Where there
is no need for the additional context info provided by the CtxDetect
dispatch function, impls. can simply pass through the common subset to
the Detect method of the analogous Detector impl.

CAVEAT: In this changeset the list of ContextualDetectors is commented-
        out. This is intended to make a clear introduction of the
        interface type prior to introducing any implementations of it.

A forthcoming change will provide such wrapping for all in-tree Detector
impls, followed by the introduction of specialization for the
GitCtxDetector impl.

[0] C.f., hashicorp#268

Signed-off-by: Alan D. Salewski <ads@salewski.email>