generalize get_fixture_name_and_path
generalize get_fixture_name_and_path

It only worked for st2tests.fixtures.packs, but we need to
use it for other packs dirs as well.

StackStorm/st2,StackStorm/st2,Plexxi/st2,StackStorm/st2,nzlosh/st2,StackStorm/st2,nzlosh/st2,nzlosh/st2,Plexxi/st2,nzlosh/st2,Plexxi/st2,Plexxi/st2