L::Util: Add locate_helper_tool

Add new utility function to locate helper code.  This will allow
others to write tools and easily find them without having to set paths
or ENV variables themselves before calling lintian.

Signed-off-by: Niels Thykier <niels@thykier.net>
coll: Move coll helpers to helpers/coll

Signed-off-by: Niels Thykier <niels@thykier.net>