Module alias as instance attribute does not work

[JukkaL]

The error generated by the following program is unexpected:

import os

class C:
    def __init__(self) -> None:
        self.os = os
        self.os.chdir('/')  # Error: Module has no attribute "chdir"
        os.chdir('/')  # Ok

C()  # No runtime error

[ilevkivskyi]

Looks like just an oversight in #3435, this

if not isinstance(lval, NameExpr):
    continue

could be probably just replaced with

if not isinstance(lval, RefExpr):
    continue

plus corresponding minor changes.