Properly support different OS versions

This would be very good to have, as having the different branches is a bit of a hassle.

What's most unclear to me is how we can add that to module descriptions.

• Ideally, you would only need to specify things that are different between two OS'es. So I don't want to have one module for Ub18 and one for Ub20, if they are 90% the same, for example.
• Also, the solution should be somewhat universal w.r.t. where you can use it in the module description. (i.e. you can specify special values for OS'es at any point in a module description.
• Finally, I don't want to change the data classes too much unless necessary.

One idea that I have is:

• We have some special syntax in the JSON files to say "This value is different for OS'es", then specify a default and special values for specific OS's.
• We pre-process the JSON file (we do that anyway for comments etc), and when encountering such a symbol, we pick the one corresponding to the current OS.

It could look something like this:

Instead of (without special values)

{
  "git": {
    "url": "...",
    "default_checkout_ref": "main"
  }
}

Version 1:

{
  "git": {
    "url": "...",
    "default_checkout_ref": {
       "per OS": {
         "default": "main",
         "ubuntu_20": "ubuntu20"
       }
    }
  }
}

or Version 2:

{
  "git": {
    "url": "...",
    "default_checkout_ref": {
      "default": "main",
      "ubuntu_20": "ubuntu20"
    }
  }
}

The second one would be less "clutter" and indirection, but also less unique.

Also, we need constant ID's for different OS's, and need to document them.

@dreher @fabian.reister What do you think about that?