Allow per-version URLs instead of one single URL per package.

lib/spack/llnl/util/lang.py

lib/spack/spack/__init__.py

lib/spack/spack/cmd/create.py

lib/spack/spack/cmd/edit.py

lib/spack/spack/concretize.py

lib/spack/spack/package.py

lib/spack/spack/relations.py

lib/spack/spack/test/package_sanity.py

lib/spack/spack/url.py

lib/spack/spack/util/compression.py

lib/spack/spack/version.py

var/spack/mock_packages/callpath/package.py

var/spack/mock_packages/direct_mpich/package.py

var/spack/mock_packages/dyninst/package.py

var/spack/mock_packages/fake/package.py

var/spack/mock_packages/indirect_mpich/package.py

var/spack/mock_packages/libdwarf/package.py

var/spack/mock_packages/libelf/package.py

var/spack/mock_packages/mpich/package.py

var/spack/mock_packages/mpich2/package.py