PruneCruft: prune fatlib/

Rationale: some author may build a fatpacked script from his project and that creates a `fatlib/` directory that contains copies of CPAN modules. If that `fatlib/` is bundled in a distribution and indexed by PAUSE, that will hide the original modules on CPAN.
We want to avoid that: bundling `fatlib` by default is always a mistake. If the author really wants that, he can still use explicitely the `exception` feature of `PruneCruft`.

See andk/pause#65