sourcedr: Cleanup Python import statements

This commit removes `from ... import *` statements, refines some
`from ... import ...` statements, and removes several unused imported
names.

Test: ./sourcedr/functional_tests.py
Test: ./sourcedr/preprocess.py
Change-Id: Iabdb8e062e3beebe0bcaffdb8703e84e1b9a0eee
4 files changed