[Extensions] Add tests for loading extension with multiple entry points

This test verifies that the extension is loaded even if the entry point
is in a different object than the one implicitly created by the extension
name.
[Extensions] Verify that the entry points are valid names

Also make sure that there aren't any extension already registered
with clashing names (or entry points).

We re-use the function that used to validate extension names to validate
entry points as well, so we rename it to something more appropriate.
[Extensions] Add tests for conflicts between extensions

This adds for when an extension adds an entry point for an object
that belongs to another extension, either the extension name or
via entry points.