Port twitter/commons to the pantsbuild/pants build system.

The {src,tests}/python/twitter/pants trees are conceptually deleted
but only the BUILD files in those trees really deleted to keep this
change reviewable.  A quick follow-up will delete those trees.

The custom goal installs have been moved to the pants plugin system
and the ./pants & venv scripts as well as pants.ini have been updated
as a result.

2 sets of additional plugins have been factored up from BUILD files to
the new plugin system:
+ commons_version() replaces the boilerplate VERSION file slurping
+ args-apt ResourceMapper handles the @Cmdline Args apt generated
  resource files from the com.twitter.common#args package hosted in
  this repo.  Since any users of that lib that use pants will need this plugin
  its exported as an sdist for re-use.

There are also alot of small BUILD file changes to catch up to new
syntax requirements including:
+ eliminate inline jar and python_requirement dependencies
+ segregate jar_library jars and dependencies lists
+ wrap python_requirements in python_requirement_libraries
+ use explicit resources lists in jvm targets
+ forgo the use of __file__ (still supported but deprecated)

https://rbcommons.com/s/twitter/r/681/