We were requiring the facet type to be complete in two different places, but this was not necessary. Now we only require it to be complete in a single place when checking the definition. If it was required earlier for a rewrite constraint, it would have been already checked and diagnosed in member lookup for the rewrite.

In the process reorganize and rename code to help clarify the relationships and behaviour between handle_impl.cpp and impl.cpp.

Move the diagnosing of extend impl T as to the handler of the T, similar to where it's diagnosed for require decls. This helps simplify the code, avoiding a bunch of looking through parse nodes for specific types. Make them both handle errors in the self type in a similar manner, avoiding a second diagnostic.