_stbt.core._find_path: Ignore callers in _stbt installation directory
_stbt.core._find_path: Ignore callers in _stbt installation directory

Doesn't make any difference now, but in future we might add images to
`$libexecdir/stbt/_stbt/` (just like we install non-python files like
`x-key-mapping.conf`, `xorg.conf.in`).

LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester,stb-tester/stb-tester,LewisHaley/stb-tester