Fix missing search&replace of .buildfile -> .build_file

Find cases of .buildfile and replace with .build_file.  Running python tests with coverage was broken.

Testing Done:
Ran python tests with PANTS_PY_COVERAGE=1

Reviewed at https://rbcommons.com/s/twitter/r/593/
rm java/examples/pingpong and many of its 3rdparty deps

rm {src|tests}/java/com/pants/examples/pingpong

rm -r 3rdparty/jvm/com/

3rdparty/BUILD -= ['guice', 'easymock']

3rdparty/BUILD -= ['mockito'] <- pingpong didn't use this; 'git grep' says nothing else did either

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/28428093

Reviewed at https://rbcommons.com/s/twitter/r/596/
Payloads with sources now give more useful errors for nesting

Because the creation of the Payloads happens later in the process
(rather than during the build file parsing), we still seem to lose track
of which file the error occurred in... is there a solution for that?

- Adds validation to SourcesMixin and switches it to inheritance with
  SourcesPayload
- Adds a Payloads test file

Reviewed on https://rbcommons.com/s/twitter/r/586/
I don't have much idea (hehehe) what I'm doing, but it is working more than it was before.

Idea files are now actually generated and IntelliJ is opened. Codegen also appears to work now, after I moved some code out of ide_gen's __init__ and into its execute(), so that the code actuall gets generated before its called for. External jars work now. I've been testing it out on the jaxb examples and tests.

The code also currently has a mess of print statmements and commented out debugger invocations, as I've been trying to decipher how the old system worked and how the new system is supposed to work.