Gazelle: detect .go sources generated by custom rules and macros

[jayconrod]

Gazelle currently detects .go files mentioned by name in the out and outs attributes of rules in BUILD files. This is helpful but incomplete. If a project uses a rule that implicitly produces a .go file or uses a macro that hides the .go file name, Gazelle won't see it and won't include the files.

One possible solution is to ask Bazel what sources are produces by all rules in a package. This could be done with bazel query or using a custom aspect. This process may be slow if the Bazel server is not already running, so it may not be appropriate to run this from an editor save hook.

Another approach would be to add some custom directive to Gazelle, telling it what the generated .go files will be for a rule or macro of a given name. This would be faster, but it would require some developer customization.

[jayconrod]

Migrated to bazelbuild/bazel-gazelle#14