update doco; goal() is obsolete

Reviewed at https://rbcommons.com/s/twitter/r/931/
Moved CucumberTest into testprojects

Testing Done:
ci passed.

Bugs closed: 532

Reviewed at https://rbcommons.com/s/twitter/r/948/
Re-enable filedeps - it was using a now-gone .expand_files() API.

This maintains a hack with special knowledge of the JvmApp target and
adds one more to deal with ScalaLibrary.java_sources.

Both of these hacks have unit-test coverage to help back up future
refactors away from these hacks.

Testing Done:
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/:all

CI went green here: https://travis-ci.org/pantsbuild/pants/builds/33652015

Bugs closed: 437, 522

Reviewed at https://rbcommons.com/s/twitter/r/939/
docs: add pages w/not much content (yet), howto_plugin, howto_customize

howto_plugin: how to write a Pants plugin

howto_customize: how to write a Pants customized for your org

So far, neither of these pages has much content. But I hope we'll
be better about capturing some of this info if we have a place to put it.

Reviewed at https://rbcommons.com/s/twitter/r/938/
Fix sloppy commit from https://rbcommons.com/s/twitter/r/901/

Testing Done:
Did a test publish from a Twitter monorepo.

Reviewed at https://rbcommons.com/s/twitter/r/940/
bad troubleshooting instructions: kicking 'em when they're down :-(
But that's what we're doing:

./pants -ldebug goal goals
Invalid command: -ldebug

Reviewed at https://rbcommons.com/s/twitter/r/953/