Fix describe_class_methods on non-mergeable DFS mro

Fix describe_class_methods on non-mergeable DFS mro

Instead of trying to deduplicate - simply track which methods are locally
defined, and use that info combined with a reverse ISA-per-select-MRO to
build the final stack.

As a result the code is even more efficient and can now deal with real-life
insane hierarchies like:

https://metacpan.org/source/MJFLICK/DBIx-Class-Bootstrap-Simple-0.03/lib/DBIx/Class/Bootstrap/Simple.pm#L93