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.

Unit tests pass.
Tested this extensively with the Diff Tool scanning.