Merge branch 'features/yaml-spec' into features/optional-deps

lib/spack/external/__init__.py

lib/spack/external/yaml/LICENSE

lib/spack/external/yaml/README

lib/spack/external/yaml/__init__.py

lib/spack/external/yaml/composer.py

lib/spack/external/yaml/constructor.py

lib/spack/external/yaml/dumper.py

lib/spack/external/yaml/emitter.py

lib/spack/external/yaml/error.py

lib/spack/external/yaml/events.py

lib/spack/external/yaml/loader.py

lib/spack/external/yaml/nodes.py

lib/spack/external/yaml/parser.py

lib/spack/external/yaml/reader.py

lib/spack/external/yaml/representer.py

lib/spack/external/yaml/resolver.py

lib/spack/external/yaml/scanner.py

lib/spack/external/yaml/serializer.py

lib/spack/external/yaml/tokens.py

lib/spack/spack/__init__.py