Support multiple packages in the same directory

[jayconrod]

(Originally bazel-contrib/rules_go#315)

We should consider supporting projects with multiple Go packages in the same directory. "go build" does not allow this, but it is fairly common within Google, and some projects being open-sourced by Google have expressed interest in this.

#5 (naming rules after Go packages) is probably a prerequisite.

[ashi009]

It has become a lower priority FR for me atm.

The most critical thing for all Bazel go project is to maintain the compatibility with the standard go toolchain — that’s the way to get out of box code completion, in-line document look up etc.

Unless we have a better approach for solving that, I don’t think this worth the effort. Of course it will be useful when open sourcing a google project.

[mariusgrigoriu]

How should we import repositories that have multiple packages in one directory? For example go_repository doesn't work with https://github.com/rivo/uniseg because the directory has a package uniseg and package main.

Can we specify which package to use for the build file?

[nvachhar]

I just ran into the issue with https://github.com/rivo/uniseg. It would be nice if we could specify the package we cared about