Move dexterity migrators into their own module

As @davisagli points out here[0], the upgrade step will fall over if there's no archetypes around. Move code out into it's own module.

This could do with testing with a real buildout, but my attempts of getting rid of Archetypes from coredev didn't go so well. Anyone know one?

[0] https://github.com/plone/plone.app.contenttypes/commit/2cd5c012c2383ab37e3c17fc37a92f806b8d47f2#commitcomment-4707719