* refactor(flake): Use a separate flake for hackage sources

This will be easier to update, and pass them as one dictionary.

* flake: support for GHC 9.2

I've naively copied most of the special cases for `GHC 9.0.1`.

- `nix develop .\#haskell-language-server-921` gives me a shell in
  which I'm able to `cabal build`
- `nix build .#haskell-language-server-921` builds HLS binary. I was
  able to use it in a proprietary codebase (by importing the flake) and
  it works fine.
- some plugins are disabled, `brittany`, `stylishhaskell`, `hlint`,
  `haddockComments`, `alternateNumberFormat`. I didn't fight too much to
  fix this because I'm not using them. I'll do an effort in the future
  to fix them.
- `nix develop .\#haskell-language-server-921-dev` does not starts, I
  need to investigate.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
Co-authored-by: Javier Neira <atreyu.bbb@gmail.com>