gh-98886: Fix issues with dataclass fields with special underscore names (#102032)

This commit prefixes `__dataclass` to several things in the locals dict:
- Names like `_dflt_` (which cause trouble, see first test)
- Names like `_type_` (not known to be able to cause trouble)
- `_return_type` (not known to able to cause trouble)
- `_HAS_DEFAULT_FACTORY` (which causes trouble, see second test)

In addition, this removes `MISSING` from the locals dict. As far as I can tell, this wasn't needed even in the initial implementation of dataclasses.py (and tests on that version passed with it removed). This makes me wary :-)

This is basically a continuation of #96151, where fixing this was welcomed in https://github.com/python/cpython/pull/98143#issuecomment-1280306360