Return more specific QNames for assignments

[zsol]

Summary

When scope.get_qualified_names_for() is called with a node that's an Assignment, return the qualified name for that node instead of all the assignments for the same name.

For example:

if False:
  def f(): pass  # {"f"}
else:
  from a import f  # {"a.f"}

f()  # {"f", "a.f"}

Before this PR, all three nodes had the same set of qnames returned ({"f", "a.f"}).

Test Plan

Will add tests before merging.

[jimmylai]

I like this change which provides more accurate and useful names.
Please also add test cases and the document.

[zsol]

While working on this I discovered that QualifiedNameProvider didn't provide names for import statements, so I added this in the commits above. This caused a completely unrelated RenameCommand to break because it assumes Name nodes inside imports don't have a qualified name (for example here).

I'll try and fix the codemod, but may decide it's more trouble than worth and just leave the original behavior for imports:

if False:
  def f(): pass  # {}
else:
  from a import f  # {"a.f"}

f()  # {"f", "a.f"}