Resurrect thin intellij project changes

Review Request #818 — Created Aug. 1, 2014 and discarded — Latest diff uploaded

johanoskarsson, jsirois, patricklaw, stuhood, travis, zundel
Travis made this change a while ago -

It got lost in the big refactor. Resurrecting it again.

From a previous email (from Travis), for context:

Hey all -

Today I experimented with creating a "thin" IntelliJ project file. Indexing goes from 4min --> 2m40s, or ~33% faster.

The general strategy is rather than having one content root set to buildroot, we have a content root per target. This restricts the files IntelliJ looks at to just the stuff actually used by the target you generated a project for. My "sketch" is and if we like this approach I can clean things up.

To summarize, with minimal effort we can:

* Reduce project indexing time by ~33% (4m --> 2m40s) by limiting the stuff intellij looks at.

Local CI passes. 
Travis ->
Generated idea project:
$PANTS_DEV=1 ./pants goal idea src/scala::

testing this on internal Twitter projects to ascertain indexing time improvements.