Filter stdlib optionally

It could be useful to filter or tag Go stdlib files in the results.
See also:
- https://github.com/bazelbuild/bazel-gazelle/blob/906a322fe7425a23b5addf0ba8f2f5aa3efe4a29/language/go/std_package_list.go#L17
- https://github.com/bazelbuild/bazel-gazelle/blob/906a322fe7425a23b5addf0ba8f2f5aa3efe4a29/language/go/gen_std_package_list/gen_std_package_list.go#L16
- https://pkg.go.dev/std