Move AllowMissingDependencies check from PathsForSource to PathForSource

This reapplies Id7925999a27ea75a05e9301bbf1eb9f9a6bc4652 with
additional fixes to not use PathForSource in PathForModuleSrc.

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.

PathForModuleSrc uses PathForSource, which causes too many
globs (>50k).  The AllowMissingDependencies check doesn't make
much sense for PathForModuleSrc, since we already know the
project containing the definition of the module exists, we can
expect its local source files to exist.  Use pathForSource and
do an manual existence check instead.

Test: paths_test.go
Test: m ALLOW_MISSING_DEPENDENCIES=true
Change-Id: If1690a708393964d3030cb908beaf7b6897c0084