Fix missing genrule srcs and tools with ALLOW_MISSING_DEPENDENCIES=true

Set the location label for missing srcs and tools to avoid
nonsensical errors when parsing the command.

Test: genrule_test.go
Test: paths_test.go
Test: unbundled branch with missing framework-res module needed by robolectric genrule
Change-Id: I9c1f1cd82a80f048c0e903b8e93910b1ae34b0b1