fixed completion and tests

to use standart file references files should be in a project. in case of Pants we don't include them before they appear in a BUILD file that's why I've added a custom completion and didn't use ReferenceProvider.

also fixed running completion test with community python plugin. and add some tests for tab comletion.

Testing Done:
tests are green

Reviewed at https://rbcommons.com/s/twitter/r/1120/