Mark mangled globals as TypeAlias

[Avasam]

Closes #600

• Mark top-level mangled identifiers as TypeAlias. (Y026)
• Change the top-level mangling prefix from global___ to Global___ to respect Y042 naming convention.

Refs https://github.com/PyCQA/flake8-pyi/blob/main/ERRORCODES.md#list-of-warnings

[nipunn1313]

mine works. What's the trouble you're seeing?

[Avasam]

Just some initial setup stuff I had to work through. Like having my files in CRLF (because I cloned from Windows with autocrlf on... oops) messing up the script, then figuring out how to install and use pyenv. But I think I got it working now.

[Avasam]

Ah, if it ends end raising Y042 instead, I don't know if that's much better. Maybe a little bit since they don't seem to exist at runtime
Speaking of which, what are they exactly, the global___s ? It seems they can only be used for typing in TYPE_CHECKING context?

Module level identifiers are mangled and aliased so that they can be disambiguated
from fields/enum variants with the same name within the file.

Wouldn't they be better prefixed __ ? (to actually name-mangle and private them, would pass stubtest, and would also respect Y042 I believe)

[Avasam]

And now unused type aliases are (rightfully) detected by Y047 ^^"
I feel like that's a nice-to-fix for another time, I'll reintroduce the stubtest entry and simply rename global___ to Global___

[nipunn1313]

Yep this all seems reasonable! Prefixing with __ also sounds great instead of Global__, but can save that for another day. Thanks for the contribution!

I'll try to tie up a few more fixes/updates and then release.