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.

Bug: 153485543
Test: all soong tests
Change-Id: Ia8a8fd965a338d0645b3721314bf91f50146ad21
Merged-In: Ia8a8fd965a338d0645b3721314bf91f50146ad21
(cherry picked from commit 5e6a7979)