sage_setup: Do not fail if sage.env, sage.* are not available #29924
Description
... so that it can be used as a build system for the most fundamental distributions of parts of sagelib such as sage-objects, sage-categories without artificially including sage.env there (which is still done as of #29865).
This will require taking care of some dependencies:
We remove some dependencies on sage.* that are still in sage_setup:
./library_order.py:77:from sage.env import cython_aliases
./command/sage_build_cython.py:21:from sage.env import (SAGE_INC, cython_aliases, sage_include_directories)
... by handling the case that the functions sage_include_directories, cython_aliases cannot be imported from sage.env
... and removing the use of SAGE_INC by using instead the environment variables set in sage-build-env-config (this is #29855; see also #29711).
Harmless dependencies on sage.*:
./docbuild/ ........
(will be invoked later, when sage.* is installed)
./optional_extension.py:22:from sage.misc.package import list_packages
./optional_extension.py:44: from sage.misc.package import is_package_installed
(no longer used, only kept around in case user packages use it)
./autogen/interpreters/__main__.py:4:from sage.env import SAGE_SRC
(only used when invoked interactively)
./command/sage_install.py:28: from sage.repl.ipython_kernel.install import SageKernelSpec
(just need to make sure that `sage_setup.command.sage_install` is only used by the distribution that installs `sage.repl.ipython_kernel`)
(split out from #29847)
Depends on #33812
Component: build
Keywords: sd111
Branch/Commit: u/mkoeppe/sage_setup_without_sage_env_etc @ cb16b91
Issue created by migration from https://trac.sagemath.org/ticket/29924