Merge branch 'features/python-modules' into develop

lib/spack/llnl/util/filesystem.py

lib/spack/llnl/util/lang.py

lib/spack/llnl/util/link_tree.py

lib/spack/spack/__init__.py

lib/spack/spack/build_environment.py

lib/spack/spack/cmd/__init__.py

lib/spack/spack/cmd/activate.py

lib/spack/spack/cmd/clean.py

lib/spack/spack/cmd/deactivate.py

lib/spack/spack/cmd/extensions.py

lib/spack/spack/cmd/find.py

lib/spack/spack/cmd/location.py

lib/spack/spack/cmd/uninstall.py

lib/spack/spack/directory_layout.py

lib/spack/spack/hooks/__init__.py

lib/spack/spack/hooks/extensions.py

lib/spack/spack/modules.py

lib/spack/spack/package.py

lib/spack/spack/packages.py

lib/spack/spack/relations.py