stubgen.py: handle method aliases similarly to function aliases

[wojdyr]

Added support for method aliases (in the same class):

A.attr("old_name") = A.attr("new_name")

will generate in class A in pyi:

old_name = new_name

[wjakob]

What if the method is a function or a method from another class? It seems to me like the name of the initializer must have the __qualname__ if it is different from where it is being inserted.

Also, could there be issues when the method references something that hasn't been declared yet? What if the stubs say

class A:
    f = A.g
    def g(self): ...

This would be an error in Python. Stubs are sometimes a bit more forgiving, but I'm wondering if it is the case here.