This is a cleaned up an simplified version of a portion of the work originally done in #22510. This ticket adds a few new features that I felt were fundamentally inseparable (in part due to peculiarities of specific packages meaning that uninstallation could not be supported without some additional work).

1. Adds a new script sage-spkg-uninstall which is invoked like sage-spkg-uninstall <pkgname> to uninstall a single package from $SAGE_LOCAL (this also allows uninstalling standard packages, though we might prefer to prevent that by default). This script handles all matters of package uninstallation, including the final step of removing the package stamp file from under $SAGE_SPKG_INST. There are two different ways a package can be uninstalled:

   a. "Modern" uninstallation: if a package has staged-installation support (i.e. has been updated to support Install packages in temporary root before copying to $SAGE_LOCAL #22509), this means that its stamp file contains a manifest of all files installed by that package (minus any files created by the package's spkg-postinst script). Therefore "modern" uninstallation consists mainly of looping over those files and removing them from $SAGE_LOCAL. Any directories left empty by this process are also removed.

   b. "Legacy" uinstallation: many (though not all) packages have in their spkg-install script some code to perform an ad-hoc uninstallation of any previous versions of the package before installing the new version (this was sometimes necessary in particular for old versions of libraries). Support for this functionality is retained in order to properly support upgrades from older versions of Sage, where most packages may not support "modern" uninstallation.

   However, in order to support this properly, the legacy uninstall code for those packages should be moved out of their spkg-install scripts and into a new spkg-legacy-uninstall script which is called by sage-spkg-uninstall for these packages. This ticket includes one such example for demonstration purposes, adding an spkg-legacy-uninstall for brial.

   In the future we may be able to deprecate, and ultimately remove these scripts. In the meantime all existing packages with uninstall code in their spkg-install should be updated (see followup ticket Add spkg-legacy-uninstall scripts for most standard packages #25140).

2. Another feature this adds which is handled by sage-spkg-uninstall, but with some required support from the sage-spkg script, are per-package spkg-prerm and spkg-postrm scripts that are run just before and just after a package is uninstalled.

   This is needed in particular for some packages (namely pkgconf and widgetsnbextension) which really can't be removed properly without some pre/post-removal steps. Other packages will need this as well, but less crucially. I haven't found a specific use case yet for spkg-postrm, but it's included for symmetry.

   Because these scripts are run during package uninstallation--of packages that are already installed--the scripts themselves need to be installed somewhere (e.g. Debian supports a similar feature, and keeps the scripts under /var/lib/dpkg somewhere). In this case a new path SAGE_SPKG_SCRIPTS is added under $SAGE_LOCAL/var/lib/sage/scripts by default. Any package-specific prerm and postrm scripts are stored there. After the package is uninstalled the scripts are removed as well.

3. The Makefile is updated so that the <pkgname>-clean rules for most packages call sage-spkg-uninstall, so that the packages are really thoroughly cleaned (rather than just removing their stamp files).

Depends on #24645
Depends on #25039

Component: build

Keywords: uninstall

Author: Erik Bray

Branch/Commit: 7a81e70

Reviewer: Julian Rüth

Issue created by migration from https://trac.sagemath.org/ticket/25139