remove inappropriate transitive dependencies by splitting conformance into a separate module

[bcmills]

#804 reports an error with go get -u for this module.

The problem arises through a large transitive dependency graph, introduced via a dependency on google.golang.org/genproto (and eventually google.golang.org/grpc and its wide net of dependencies) in the conformance/internal/conformance_proto package:

~/src/github.com/golang/protobuf$ go mod why -m google.golang.org/genproto
# google.golang.org/genproto
github.com/golang/protobuf/conformance/internal/conformance_proto
google.golang.org/genproto/protobuf/field_mask

However, the conformance subtree doesn't even build using cmd/go: it has its own Makefile and .sh test driver. It should not be included in the main module, and then the inappropriate grpc dependency will be removed.

That will also help to address golang/lint#436.

CC @dmitshur @thepudds

[dsnet]

We should just delete the conformance test. It hasn't been run in forever, and most of the new shiny stuff is in v2 anyways (along with a eventually better written conformance test).

[dsnet]

Back-tracking a bit, it seems that the real issue is that field_mask is in genproto. It's entirely reasonable that this repo needs to depend on field_mask again in the future since it's a well-known type with special semantic meaning.

Thus, we should really move field_mask over to this repo.

[dsnet]

Related issues with field_mask are #745 and #398 and #218.

[bcmills]

That's fair, but I think for now let's stop the bleeding by cutting out the conformance module. Then we can do the rest of the cleanup as time permits.

[dsnet]

Fixed by #808.