Add extensions command.

lib/spack/llnl/util/link_tree.py

lib/spack/spack/cmd/extensions.py

lib/spack/spack/directory_layout.py

lib/spack/spack/package.py

lib/spack/spack/packages.py

lib/spack/spack/spec.py

var/spack/packages/py-basemap/package.py

var/spack/packages/python/package.py