The contents of the stamp files that get written to $SAGE_SPKG_INST by spkg-install have a somewhat ad-hoc format. This updates the format to use JSON.

This change, and in particular the use of JSON, is motivated by #22510. The idea here is I would like to write a list of installed files for a package, along with any other metadata about that package to the same file, so this requires some amount of structure.

This could be accomplished with a simpler format easily, but I wanted to avoid using an ad-hoc format.

The downside to using JSON is that it is harder to parse with simple shell tools. But as far as I know there are not currently any tools that consume the contents of these files. And if such tools are written they can use Python or another language with JSON support.

Alternatively the files could be YAML-formatted--simple YAML files can be simpler to parse than JSON. But if want to parse it from Python we would probably want to include pyyaml as a dependency.

CC: @jdemeyer

Component: build

Author: Erik Bray

Branch/Commit: a53027e

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/23096