The method is now called "use_repositories" and
makes it clear in the docstring that it accepts
as arguments either Repo objects or paths.

Since there was some duplication between this
contextmanager and "use_repo" in the testing framework,
remove the latter and use spack.repo.use_repositories
across the entire code base.

Make a few adjustment to MockPackageMultiRepo, since it was
stating in the docstring that it was supposed to mock
spack.repo.Repo and was instead mocking spack.repo.RepoPath.