Please v6.0.3

First release of the v6 series for Please. New features include:

• A linter to identify potential problems in BUILD / build_defs files
• Support for generic config properties in .plzconfig which makes extensions nicer to write
• Several cases of nondeterminism fixed
• Outputs are now ordered canonically before hashing
• maven_jar is split into separate parts so the source rules are still there but other parts of the system can get away without dealing with them.
• cgo_library's package argument has changed to out to match other rules
• cc_embed_binary should finally work on OSX, but requires nasm there.
• Versions in .plzconfig are parsed as valid semvers

Plus a bunch of smaller things. As usual any new bugs are unannounced here.