Merge pull request #378 from mathstuf/dep-types

lib/spack/docs/packaging_guide.rst

lib/spack/spack/__init__.py

lib/spack/spack/build_environment.py

lib/spack/spack/cmd/fetch.py

lib/spack/spack/cmd/info.py

lib/spack/spack/cmd/mirror.py

lib/spack/spack/cmd/module.py

lib/spack/spack/cmd/package-list.py

lib/spack/spack/cmd/test-install.py

lib/spack/spack/concretize.py

lib/spack/spack/database.py

lib/spack/spack/directives.py

lib/spack/spack/directory_layout.py

lib/spack/spack/graph.py

lib/spack/spack/modules.py

lib/spack/spack/package.py

lib/spack/spack/spec.py

lib/spack/spack/test/cmd/test_install.py

lib/spack/spack/test/concretize.py

lib/spack/spack/test/mock_packages_test.py