patch pkg_resources while the working_set is activated.

i was unaware of the existence of pkg_resources.working_set, but this is
apparently what a number of pkg_resources methods rely upon to handle
distribution finding and entry point management.  how i managed to miss this
for so long escapes me.  hooray for no documentation whatsoever.

this changes:
  1. pulls the PEX_INTERPRETER / no-entry-point code into PEX.execute_interpreter
  2. makes PEXEnvironment return the WorkingSet following .resolve
  3. adds a pkg_resources patching contextmanager to patch working_set and its
     associated methods for the duration of the PEX execution

for more background on the working_set patching, see:
https://bitbucket.org/pypa/setuptools/annotate/8e8c50925f18eafb7e66fe020aa91a85b9a4b122/pkg_resources.py?at=default#cl-2752

Testing Done:
mba=twitter-commons=; ./pants py -r rbtools==0.5.7 -e rbtools.commands.main:main -- post -o -g

Reviewed at https://rbcommons.com/s/twitter/r/52/