go_path: support go:embed of generated files (#3285)

When embedded files are generated, relativize fails because the file
path starts with the output directory. Without the change in path.bzl,
the test case fails as follows:

Error in fail: Path 'bazel-out/darwin-fastbuild/bin/tests/core/go_path/pkg/lib/renamed_embedded_src.txt'
is not beneath 'tests/core/go_path/pkg/lib'
5 files changed