Move AllowMissingDependencies check from PathsForSource to PathForSource

PathsForSource was handling the AllowMissingDependencies case, but
PathForSource was not.  Refactor PathForSource and
ExistentPathForSource, and add logic to PathForSource to fall back
to behavior similar to ExistentPathForSource when
AllowMissingDependencies is set.

Test: paths_test.go
Change-Id: Id7925999a27ea75a05e9301bbf1eb9f9a6bc4652