_stbt.core: Rename _find_path to _find_user_file
_stbt.core: Rename _find_path to _find_user_file

It explicitly ignores the `_stbt/` directory. We wouldn't want to
confuse it with our ubiquitous `_find_file` private helper functions.

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