Partial commit of more packages.

README.md

lib/spack/llnl/util/filesystem.py

lib/spack/spack/build_environment.py

lib/spack/spack/cmd/create.py

lib/spack/spack/package.py

lib/spack/spack/util/executable.py

var/spack/packages/clang/package.py

var/spack/packages/llvm-compiler-rt/package.py

var/spack/packages/llvm-lld/package.py

var/spack/packages/llvm/package.py