Better install strategies

Right now, HLS >= 1.7.0.0 and all GHC versions get installed in

• ~/.ghcup/<tool>/<ver> and then symlinks into ~/.ghcup/bin

That makes it easy to discover which tools are installed.

However, cabal/stack get installed as

• ~/.ghcup/bin/<tool>-<ver> (only ~/.ghcup/bin/<tool> is a symlink)
• under XDG: ~/.local/bin/<tool>-<ver> (that already causes some problems with similarly named binaries like cabal-plan)

It would be better to use the ~/.ghcup/<tool>/<ver> scheme for all tools, regardless of whether they install a lot of files or not.

The main issue here is how to be backwards compatible.