Allow tests to bypass PathForSource existence checks

Forcing every test to specify every file it wants to pass to
PathForSource or PathForModuleSrc is painful to maintain and
doesn't add any value.  Allow tests to reference paths through
PathForSource and PathForModuleSrc without specifying them in
the mock FS.

Test: all soong tests
Change-Id: Ia8a8fd965a338d0645b3721314bf91f50146ad21