cmd/gazelle

This is essentially a copy of the files in bazel-contrib/bazel-gazelle/cmd/gazelle.

To upgrade gazelle, one must:

• Compare changes in the source repo@version to the files here. It's easiest to just copy over each file and see where the diffs are. Make sure langs.go includes github.com/stackb/rules_proto/language/protobuf.
• Since the proto_gazelle.bzl rule uses @bazel_gazelle//internal:gazelle.bash.in, changes there must remain compatible with proto_gazelle. Look at the diff there and make sure the _proto_gazelle_impl is satifying the needs of that template.
• Remember that this cmd/gazelle must be buildable via the standard go toolchain (see proto_repository_tools.bzl). All deps must be in the vendor tree.