Refactor the pkginfo code and fix issue with go_libraries in subrepos

[Tatskaari]

There was an issue in subrepos created via go_repo where we'd include the pkgpath in $PKGDIR. The assumption that importPath := filepath.Join(module, pkgDir) where pkgDir is $PKG_DIR, doesn't hold for this case. We'd end up with github.com/owner/module/pkg/src/github.com/owner/pkg/src as the import path.

This PR refactors the code so we have different entry points for go_library and go_module that are more explicit about these arguments. I have done some spot checking, comparing the results from //third_party/go:std, //tools/please_go/packageinfo and ///third_party/go/github.com_stretchr_testify//assert and they all look good. One small thing is we're now returning empty packages for directories that contain only test sources:

  {
    "ID": "cmd/api",
    "Name": "main",
    "PkgPath": "cmd/api"
  },

This is probably harmless but I think the package driver should include test sources somehow. The fact that it doesn't in probably a bug.