Add one file build/pkgs/*/repology.txt per spkg

[slel]

We add a files repology.txt for each "SPKG" with its
name or names on repology.org.

This can save time when

• adding package information for an already supported or new distro
• checking for new versions of packages

We make ./sage -info packagename print out
the resulting links to repology.

Thus they are also formatted as part of the SPKG documentation
that goes into the reference manual (#29655).

Depends on #29655
Depends on #30944

CC: @mkoeppe @slel @dimpase @seblabbe

Component: porting

Keywords: repology

Author: Samuel Lelièvre, Matthias Koeppe

Branch/Commit: c590a13

Reviewer: Matthias Koeppe, Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/31114

[slel]

comment:1

If that sounds interesting, I can add that information.

[mkoeppe]

comment:2

Good idea. The package name is probably enough

[slel]

Branch: u/slelievre/repology

[slel]

Commit: 2d9166c

[slel]

Description changed:

--- 
+++ 
@@ -1,6 +1,7 @@
-It would be nice to have a file `repology.txt` for each "spkg",
-with the package's name on [repology.org](https://repology.org)
-and/or the url for getting information about this package there.
+We add a files `repology.txt` for each "SPKG" with its
+name or names on [repology.org](https://repology.org).
 
-This would save time when adding package information for any
-already supported or new distro.
+This can save time when
+
+- adding package information for an already supported or new distro
+- checking for new versions of packages

[slel]

New commits:

2d9166c

t-31114 Add repology package information

[mkoeppe]

comment:5

Great! I will add some code to format it as part of our generated documentation

[mkoeppe]

Changed branch from u/slelievre/repology to u/mkoeppe/repology

[mkoeppe]

Dependencies: #29655

[mkoeppe]

Changed commit from 2d9166c to f00af7d