go_repository takes Gazelle directives

[linzhp]

As discussed in #598, this PR make go_repository take a list of Gazelle directives to be written to the root level build file.

This PR supercedes #597 and #598

[jayconrod]

Please also add something to internal/go_repository_test.go to verify this works.

[jayconrod]

We should only write this if ctx.attr.gazelle_directives is set.

Ideally we would append to the file rather than replacing it if it already exists. I don't think there's a simple way to do that though. Perhaps setting this attribute should be an error if the file exists.

[linzhp]

If build_file_generation = "auto" and build files exist in the repo, Gazelle will not run and no Gazelle directive is needed. We can just ignore them. If build_file_generation = "on", go_repository will override the existing build files if there is any. Overriding the root build file with Gazelle directives feels consistent with the behavior. Do you think it necessary to report an error for this case?

[jayconrod]

Thanks! Looks good, but I've pushed one small change, contradicting my earlier feedback: directives should probably be called build_directives instead, since there are several other attributes related to build files that start with build_.