Added support for virtual dependencies ("provides")

lib/spack/spack/cmd/spec.py

lib/spack/spack/concretize.py

lib/spack/spack/package.py

lib/spack/spack/packages/__init__.py

lib/spack/spack/relations.py

lib/spack/spack/spec.py

lib/spack/spack/test/concretize.py

lib/spack/spack/test/mock_packages/callpath.py

lib/spack/spack/test/mock_packages/mpileaks.py

lib/spack/spack/test/spec_dag.py