Removed `which` dependency, now using proper shell builtin.

- `which` isn't installed by default on all distros.
- `which` doesn't resolve exactly like the shell, potential bugs.
- `which` unnecessary spawns a new process.

Change-Id: Ie89ea78b6f4cfb9c5415e889ec0393924870d883
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>