Move jsonschema files out of `config.py` to their own package.

lib/spack/spack/config.py

lib/spack/spack/schema/__init__.py

lib/spack/spack/schema/compilers.py

lib/spack/spack/schema/mirrors.py

lib/spack/spack/schema/modules.py

lib/spack/spack/schema/packages.py

lib/spack/spack/schema/repos.py

lib/spack/spack/schema/targets.py