[possibly-undefined] Fix loop misconceptions

[NMertsch]

This PR fixes two misconceptions in the current possibly-undefined handling of for and while loops.

for loop: index is only defined if the body is executed

Currently, the index is always considered as defined. Counter example:

for i in []:
    pass

i  # Name "i" is undefined

for and while loop: else is executed if the body is not executed

Currently, the else block of loops is only considered as executed if there was no break. Counter examples:

for _ in []:
    break
else:
    print("executed")

while False:
    break
else:
    print("executed")

The fix basically handles both cases like if {body is executed}: ...; else: ....
This is more accurate than the current implementation, but there is still room for improvement: #14209 (comment)

Refactorings

Following the Boy Scout Rule, I added a few minor code style improvements in separate commits. I'm fine with dropping them, please see them as suggestions.

[KotlinIsland]

you would want to investigate mypy.partially_defined.PossiblyUndefinedVariableVisitor.visit_for_stmt

[KotlinIsland]

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

don't get worried that this primer is empty, it doesn't have any rules enabled. if you wanted, you could run a custom primer with the relevant rule enabled

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

CPython (cases_generator) (https://github.com/python/cpython)
+ Tools/cases_generator/generators_common.py:89: error: Name "tkn" may be undefined  [possibly-undefined]