Support loading subtasks from a different file

This is an incremental step to completely support moving
non root tasks away from task.config in All-Projects repo.
Various syntaxes were introduced to import a single task
from other files in same ref and are described in the
documentation.

Originally-Authored-By: Adithya Chakilam
Change-Id: I794e6a2b6d8c93e92e633c12823c956128e2ffe6