Y047 false negative for conditionally defined aliases

[AlexWaygood]

This caused me some confusion in python/typeshed#10177, where I expected Y047 to be emitted, but it wasn't.

Code like this currently doesn't cause Y047 to be emitted, but it probably should:

import sys
from typing_extensions import TypeAlias

if sys.version_info >= (3, 12):
    _Foo: TypeAlias = str
else:
    _Foo: TypeAlias = int

The reason is that this logic here is too naive:

flake8-pyi/pyi.py

Lines 1937 to 1939 in 5b17698

	for alias_name, alias in self.typealias_decls.items():
	if self.all_name_occurrences[alias_name] == 1:
	self.error(alias, Y047.format(alias_name=alias_name))

If an alias is conditionally defined, as above, the name of the type alias ("_Foo") will appear more than once in the file (it will appear once for each conditional definition), which leads flake8-pyi to assume that the type alias has been "used" at least once in the file that it's been defined in.