package.py : moved each specialized package to its own module file

lib/spack/spack/__init__.py

lib/spack/spack/build_systems/__init__.py

lib/spack/spack/build_systems/autotools.py

lib/spack/spack/build_systems/cmake.py

lib/spack/spack/build_systems/editable_makefile.py

lib/spack/spack/package.py

var/spack/repos/builtin/packages/astyle/package.py