stubgen: fixes and simplifications

[hamdanal]

This PR refactors name resolution in stubgen to make it simple to use and fixes a bunch of related bugs especially when run in --parse-only mode. It mainly does the following:

1. Adds a get_fullname method that resolves the full name of both NameExpr and MemberExpr taking into account import aliases
2. Use this method everywhere manual resolution was done including the decorator processors
3. Simplify decorator processing by consolidating process_decorator, process_name_expr_decorator, and process_member_expr_decorator into a single method thanks to the common name resolution using get_fullname
4. Fix occurrences of some hard-coded implicitly-added names (like Incomplete from _typeshed) by using self.typing_name() and self.add_typing_import which take into account objects of the same name defined in the file
5. Fix some inconsistencies in generated empty lines. This improves the consistency within stubgen as well as with the black code style for stubs.

Out of the 9 test cases added, only testAbstractPropertyImportAlias passes on master (added it any way because it was not tested). The other 8 all fail without this PR. The last test case testUseTypingName demonstrates point 4.. In existing test cases, only white space changes in four of the cases due to point 5. above were made.

This will allow easier additions in the future as no manual name resolution has to be made any more.

[JelleZijlstra]

There's a CI failure (apparently only when mypyc is enabled).

[hamdanal]

This turned out to be a python 3.11 failure because asyncio.coroutine was removed in 3.11 and mypy couldn't resolve the import. get_fullname now falls back to non-semantic analysis resolution when it detects a suppressed import because stubgen should still generate the stub correctly even when its run with a newer version of python

[hauntsaninja]

Thanks, this cleans up several things. I'll leave open for a couple days in case someone wants to take a second look (many lines touched, so it's easy to miss things in review).