sage.matrix: Resolve circular imports without using __init__.py #30784
Description
This is preparation for making sage.matrix a namespace package in #28925.
CC: @simon-king-jena @tscrim @tobiasdiez @videlec @kiwifb
Component: refactoring
Author: Matthias Koeppe
Branch/Commit: ea0d15a
Reviewer: François Bissey
Issue created by migration from https://trac.sagemath.org/ticket/30784