Bundle Cabal and Cabal-syntax source with cabal-install?

Perhaps we should bundle the latest `Cabal` and `Cabal-syntax` libraries with the `cabal-install` executable data files.

Then `cabal-install` can decide whether to build the latest `Cabal` library on the fly if the user's plan requires a `very new cabal version.

For example #9632 would benefit from this, if you use `--multi-repl` you get a constraint on a very new `Cabal`l library.

Some care would need to be taken to engineer things so that the new version is not picked as a matter of priority and only if explicitly needed.

My intuition is that if the packages were provided as a local+noindex repo then things would work out "just right".

I constructed an incomplete POC here (not currently using local+noindex): https://github.com/mpickering/cabal/tree/wip/bundle-cabal-package

Is this desirable to keep working on?