style: make core comply with pep8-naming

.flake8

lib/spack/llnl/util/lang.py

lib/spack/llnl/util/multiproc.py

lib/spack/llnl/util/tty/__init__.py

lib/spack/spack/architecture.py

lib/spack/spack/binary_distribution.py

lib/spack/spack/cmd/create.py

lib/spack/spack/compilers/nag.py

lib/spack/spack/compilers/xl_r.py

lib/spack/spack/config.py

lib/spack/spack/directives.py

lib/spack/spack/fetch_strategy.py

lib/spack/spack/mirror.py

lib/spack/spack/operating_systems/mac_os.py

lib/spack/spack/package.py

lib/spack/spack/platforms/bgq.py

lib/spack/spack/platforms/darwin.py

lib/spack/spack/platforms/linux.py

lib/spack/spack/platforms/test.py

lib/spack/spack/stage.py