Portions of namespace packages can be installed in separate locations.

Hence, the variable sage.env.SAGE_LIB is no longer meaningful.

Instead, we should make use of the __path__ attribute of packages, https://python.readthedocs.io/en/stable/reference/import.html#module-path, which in the case of namespace packages is an iterable.

• Use of SAGE_LIB in src/sage/misc/lazy_import_cache.py can possibly be removed after Remove expired deprecations that use lazy_import(MODULE, "*") #33308

• sage -t --installed #33407 sage -t --all-installed

• Remove use of SAGE_LIB in sage.misc #33821 Remove use of SAGE_LIB in sage.misc

• sage.env.sage_include_directories: Don't use distutils and SAGE_LIB #33137 sage.env.sage_include_directories: Don't use distutils and SAGE_LIB

SAGE_EXTCODE should be replaced by using importlib.resources (more precisely, the backport package importlib-resources). Version 5.9.0 can do as_file with directories (https://importlib-resources.readthedocs.io/en/latest/history.html#v5-9-0)

Depends on #31306

CC: @tornaria

Component: refactoring

Issue created by migration from https://trac.sagemath.org/ticket/33037