Rename packages.py to repository.py, PackageFinder to RepoPath.

lib/spack/spack/__init__.py

lib/spack/spack/cmd/repo.py

lib/spack/spack/cmd/uninstall.py

lib/spack/spack/repo_loader.py

lib/spack/spack/repository.py

lib/spack/spack/test/directory_layout.py

lib/spack/spack/test/mock_packages_test.py

lib/spack/spack/test/package_sanity.py

lib/spack/spack/test/packages.py