Refactor import-tracking to leverage existing AST bindings (#1856)

This PR refactors our import-tracking logic to leverage our existing
logic for tracking bindings. It's both a significant simplification, a
significant improvement (as we can now track reassignments), and closes
out a bunch of subtle bugs.

Though the AST tracks all bindings (e.g., when parsing `import os as
foo`, we bind the name `foo` to a `BindingKind::Importation` that points
to the `os` module), when I went to implement import tracking (e.g., to
ensure that if the user references `List`, it's actually `typing.List`),
I added a parallel system specifically for this use-case.

That was a mistake, for a few reasons:

1. It didn't track reassignments, so if you had `from typing import
List`, but `List` was later overridden, we'd still consider any
reference to `List` to be `typing.List`.
2. It required a bunch of extra logic, include complex logic to try and
optimize the lookups, since it's such a hot codepath.
3. There were a few bugs in the implementation that were just hard to
correct under the existing abstractions (e.g., if you did `from typing
import Optional as Foo`, then we'd treat any reference to `Foo` _or_
`Optional` as `typing.Optional` (even though, in that case, `Optional`
was really unbound).

The new implementation goes through our existing binding tracking: when
we get a reference, we find the appropriate binding given the current
scope stack, and normalize it back to its original target.

Closes #1690.
Closes #1790.

resources/test/fixtures/flake8_annotations/allow_nested_overload.py

@@ -0,0 +1,9 @@
+class C:
+    from typing import overload
+
+    @overload
+    def f(self, x: int, y: int) -> None:
+        ...
+
+    def f(self, x, y):
+        pass

resources/test/fixtures/pyflakes/F401_6.py

@@ -9,7 +9,6 @@ from .background import BackgroundTasks
 # F401 `datastructures.UploadFile` imported but unused
 from .datastructures import UploadFile as FileUpload
 
-
 # OK
 import applications as applications
 

resources/test/fixtures/pygrep-hooks/PGH002_0.py

@@ -5,4 +5,3 @@ from warnings import warn
 warnings.warn("this is ok")
 warn("by itself is also ok")
 logging.warning("this is fine")
-log.warning("this is ok")

resources/test/fixtures/pygrep-hooks/PGH002_1.py

@@ -2,14 +2,4 @@ import logging
 from logging import warn
 
 logging.warn("this is not ok")
-log.warn("this is also not ok")
 warn("not ok")
-
-
-def foo():
-    from logging import warn
-
-    def warn():
-        pass
-
-    warn("has been redefined, but we will still report it")