623: WIP: module id is not def id r=matklad a=matklad
This achieves two things:
* makes module_tree & item_map per crate, not per source_root
* begins the refactoring to remove universal `DefId` in favor of having separate ids for each kind of `Def`. Currently, only modules get a differnt ID though.
Co-authored-by: Aleksey Kladov <aleksey.kladov@gmail.com>