Add a function for iterating through all exe locations in the PATH.
Add a function for iterating through all exe locations in the PATH.

This introduces `rbtools.utils.filesystem.iter_exes_in_path()`, which
looks for the specified executable in each path in `$PATH`, yielding
each one that's found. This can be used to scan for multiple instances
of an executable, trying to find a specific version.

`is_exe_in_path()` now wraps this, returning just the first result.

Testing Done:
Unit tests pass.

Tested this extensively with the Diff Tool scanning.

Reviewed at https://reviews.reviewboard.org/r/12617/

reviewboard/rbtools,reviewboard/rbtools,reviewboard/rbtools