update Pants+IntelliJ docs to say the Plugin's the new hotness, link to plugin's README

Reviewed at https://rbcommons.com/s/twitter/r/891/
Fix a nasty bug when injecting target closures in BuildGraph

Just because a target has been injected doesn't mean that its
closure (and therefore its dependency edges) have been injected.
The current code misses the case where a target was injected
(but not its closure) and then has its closure injected later.

Testing Done:
Added a test that identifies the bug, which now passes (doesn't on master):
tests/python/pants_test/graph test_inject_then_inject_closure.

CI is green: https://travis-ci.org/pantsbuild/pants/builds/41270498

Reviewed at https://rbcommons.com/s/twitter/r/1337/
Sync of https://github.com/pantsbuild/pants/commit/bdeb6958135db3adcc19623a0bf7461363fbc73c