Follow-up from #29287.

Script packages are installed by special rules in build/make/Makefile.

This ticket installs them by sage-spkg instead. This enables script packages to participate in the staged installs (SAGE_DESTDIR) and uninstallation mechanisms.

This is useful for #29387 to clean up #27870/#29003/#29082, which introduced files in SAGE_LOCAL that do not belong to any package and are never uninstalled.

We also make the decision whether spkg-install is generated from a template spkg-install.in or provided as is in the source tree orthogonal to the distinction source=script vs. source=pip.
We simply generate spkg-install if spkg-install.in is present.

Ideally, when done, changing a package's source between script and normal and pip will no longer require running bootstrap.

CC: @dimpase @jhpalmieri @embray @orlitzky

Component: build

Work Issues: Move "dummy script package" code to sage-spkg

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/install_script_packages_via_sage_spkg @ 7a25d1d

Issue created by migration from https://trac.sagemath.org/ticket/29386