Ease configuring and running the builtin plugins.

[monopole]

This PR allows a user to run a builtin plugin via the transformers: or generators: field without needing to also make the .so file for said plugin available.

The PR also makes it possible to run a builtin plugin without specifying the --enable_alpha_plugins flag.

The gist of this PR is to exploit the statically linked factory functions already being created by the pluginator code generator. The PR moves a few interfaces around to avoid package cycles, and adds an enum type for the builtins, and a map associating the enums to the static plugin factory functions.

I had to modify the pluginator, the code generator that provides statically compiled factory methods for making plugins, to return interface types rather than concrete instances so that the factory methods could be invoked from generic code detecting the use of builtin plugins.

Fix #1504
Fix #1508

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[monopole]

@richardmarshall @jcassee PTAL if you have the time.

It's not as complex as it looks. The longish list of files modified is because i had to gather some interface definitions into the resmap package (which ultimately is a better place for them anyway).

[Liujingfang1]

/lgtm

[jcassee]

This is great. Treating builtin generators and external generators equally feels nice. One thing I noticed is that labels and annotations on a ConfigMapGenerator are not passed to the ConfigMap, but that never worked, right?

Not sure if you are looking for comments on the code. I never went into this part of the woods. The pluginator seems either a transitional mechanism or a stopgap, as it generates duplicate code by design? I'm not completely sure what the changes do without diving deeper, but they seems to be doing it quite simply. :-)

Any reason for choosing "builtin" as api version, and not something like "kustomize.config.k8s.io"? A conscious decision not to do version management (yet)?