Make sagelib pip-installable in a conda environment

[mkoeppe]

The goal of this ticket is to make sagelib (i.e., subdirectory SAGEROOT/src) pip-installable when all required packages are provided by the conda distribution -- just through SAGEROOT/src/setup.py and completely bypassing all sage-the-distribution scripts (top-level SAGEROOT/Makefile and SAGEROOT/build.

• Add SAGEROOT/src/environment.yml - describes all conda packages needed for installing sagelib for doing sagelib development (this is different from SAGEROOT/environment.yml introduced in #28745)
• #21569: Install src/bin/* scripts via setup.py (scripts, console_scripts)
• #21785: Installation of SAGE_SRC/ext/ in SAGE_LOCAL/share/sage/ext/ should be done by setup.py, not build/make/Makefile
• #25828: Move OptionalExtension to `features

Optional steps:

• #21559: Move sage-the-distribution scripts from src/bin to build/bin
• #21570: Move non-scripts of src/bin/ elsewhere (and also move their install location)
• #21573: Make sure src/setup.py respects --install-base and --root
• #21654: src/setup.py: Disentangle cleaning of stale installed files in build directory and in install directory
• #21678: Testsuite for src/setup.py
• #21707: Split sage-env into sage-build-env and sage-env
• #22655: Support package_data-like of non-Python resource files in Python packages
• #23747: Rename SAGE_ROOT/build to SAGE_ROOT/sage-distro, SAGE_ROOT/src to SAGE_ROOT/sagelib
• #21516: Fix sagelib sdist (src/setup.py sdist)

(see also earlier task ticket #21507: Make sagelib a pip-installable Python source package, listed on PyPI.)