Make ./pants goal targets to find all the targets aliases/types automatically instead of being hardcoded.

Currently in TargetsHelp, we hardcoded the list of target aliases to target types mapping. This RB changes it to make the discovery dynamically, by looking at all the registered aliases in build_configuration and then intersect them with all the installed target types from SourceRoot.

Testing Done:
passed: PANTS_DEV=1 ./pants goal test tests::

Bugs closed: 333

Reviewed at https://rbcommons.com/s/twitter/r/796/