glyphs2ufo: prune recursive references

I occasionally get broken Glyphs.app files that have components pointing to themselves (maybe even in bracket layers) or otherwise resulting in a loop. glyphsLib should aggressively prune them with a warning.

Start at the glyphs2ufo conversion entry point and trace how component references are traversed, including bracket layers. Reproduce the issue with a Glyphs file containing a self-referential component or loop; done means recursive references are pruned and a warning is emitted.

Written by the indexing model from the issue text.