Revert "Revert "Bug 890065 - Add a flag for optionally disabling sources.xml""

This reverts commit 612e51e0491732fd1ce9869b14b17144fdba0da6.
Revert "Bug 897559 - Use |repo manifest| to parse manifest"

This reverts commit aceff4f45ac37a30d781939fd78a252dbf7ccc7a.
Merge pull request #106 from dhylands/backout-897559

Backout 897559 (Request by RyanVM due to bustage)