[F402] Warn on loop variable shadowing module name

[ansley]

Stack from ghstack (oldest at bottom):

• [F823] Warn when variable name is referenced before assignment #277
• [F812] Warn when list comprehension redefines a name #276
• [F811] Warn on and delete double import #275
• [F403, F405, F821] Warn on undefined names #274
• [F403] Warn on star import #273
• -> [F402] Warn on loop variable shadowing module name #272
• Test rules in pyflakes directory #271

[amyreese]

Needs formatting fixes (make format or hatch run lint:fix).

[amyreese]

I think we need more variety of test cases, covering things like loops with multiple elements (or deep names), as well as some validating things like import a.b where a is the name that might be shadowed, but b isn't. Take a look at upstream pyflakes tests (search for "shadow" to find instances), and see what other cases we can cover from their examples.

[ansley]

I didn't see much in the pyflakes project (searched "shadow" and a few other keywords), but I did add more tests!