[RFC] Analyze attribute access from imports to support dead import clean up better

[jimmylai]

Per @zsol 's request, propose this change. Feedback is welcome.

In this example, the access a.b is recorded in assignment a.b and a.c. So the RemoveUnusedImportsCommand cleans up nothing.

import a.b
import a.c

a.b

It's because we only record the top level name of assignments and accesses.
https://github.com/Instagram/LibCST/blob/master/libcst/metadata/scope_provider.py#L310-L311

For example, when we see an access a.b, a can be imported or a local object.
To provide the dead import clean up support better, we can record the best assignment and access when multiple imports are available in infer_access.

https://github.com/Instagram/LibCST/blob/master/libcst/metadata/scope_provider.py#L786-L800

That means, given an access, we can check all import assignments having the same top level name (a.b and a.c) and only assign the access when it startswith the fully qualified name of the access.