allow referencing mocked classes by the class itself on top of the class name

[tcassou]

Motivation & Context

Using the class name as a reference when setting up mocks is a handy shortcut that can save a lot of import statements. However, it only works well for small projects where there are no collisions between class names. Under the hood, individual mocks are indexed by class name and method name, which makes them vulnerable to collisions.

This PR adds support for referencing mocked classes through the class itself, to resolve this limitation.
In larger projects where class names may collide, using direct references allows avoiding mock collisions; the only downside being the need to import all mocked classes explicitly.

Description

• adding tests (some_conflicting_api_test.py) to simulate such collisions
• maintaining a double index of mocks in Expect: the current one (class_name, method_name) -> mock, and a new, more robust one (class, method_name) -> mock
• leveraging either of them depending on how Expect() statements are defined (forbidding any mix inside a given test)
• adding a battery of tests to confirm the new behavior works

Breaking Changes

None: all existing tests still pass. This PR introduces new and compatible functionality.

Checklist

• Changes are tested
• Pre-release packages work as expected
• Docs are up to date (README.md, docstrings, etc)