Skip existence check for stub library files

Switch from `PathForModuleSrc` to `PathForSource`. The latter does not
check that the target path exists in the tree. This is necessary since
the prebuilt stub and header directories are not guaranteed to exist
when Soong analysis begins (Step 2a)

Build orchestrator will be responsible for putting these files in the
right place as part of Multi-tree ninja invocation (Step 4)

Test: go test ./cc
Change-Id: I27175a8440fca6bba21197b1e106a22b733da882