Fix implicit __loader__ module attribute

[nightcityblade]

Fixes #4182

Mypy now recognizes __loader__ as an implicit module attribute. It uses _typeshed.importlib.LoaderProtocol | None with the standard typeshed and falls back to object | None when import following is unavailable or the test fixtures are active.

The existing implicit-module-attributes test now covers __loader__.

Tests:

• .venv/bin/python -m pytest -n0 'mypy/test/testcheck.py::TypeCheckSuite::check-basic.test' (46 passed)
• .venv/bin/python -m pytest -n0 mypy/test/testexportjson.py (8 passed)
• .venv/bin/python -m pytest -n0 -k 'testInspectModuleAttrs' (2 passed)
• .venv/bin/python runtests.py self (341 source files checked)
• .venv/bin/python runtests.py lint (all hooks passed)
• .venv/bin/python -m mypy -c 'reveal_type(__loader__)'

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅