Make multi-distro support more modular in sage-spkg

[embray]

#29053 adds a feature to sage-spkg which prints the necessary distro-specific commands for installing a dependency of Sage with the appropriate packaging system.

This is currently hard-coded in #29053 in a case statement, but as this is simply data, I might suggest moving it out to a file that can be read by sage-spkg and easily modified or extended to add support on other distros.

This proposal is currently not opinionated on what that file would look like, other than that it should be simple (it could even a directory of small shell scripts, such as a distros.d/ so that additional distros can be added without patching).

CC: @dimpase @embray @antonio-rojas @isuruf @timokau

Component: build

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/29157

[mkoeppe]

comment:1

dup of #29146.

[dimpase]

Reviewer: Dima Pasechnik