So far this is just an illustration of what can be done with flit (#29846) and what cannot:

• Both non-src and src layouts are supported (for the latter, see WIP: allow src dir pypa/flit#260).
• Symlinks of directories or files to somewhere outside of the root are not supported.

Not intended for merging at this point.

Depends on #29846

Component: build

Branch/Commit: u/mkoeppe/install_sage_conf_using_flit @ 7ea5fab

Issue created by migration from https://trac.sagemath.org/ticket/29854