add a label resolving method

something like `str(Label("//label"))` so that rule developers have a nice method to resolve labels even if someone imports your repository in a different name.
Spotted with @damienmg during a hack session

The issue does not name a file, test, or entry point. Start by locating the Label abstraction and existing label-resolution behavior in the repository, then determine the expected behavior for str(Label("//label")) when the repository is imported under another name. Done means the method resolves labels correctly and has coverage for that import scenario.

Written by the indexing model from the issue text.