Safety concerns with recursive deletion and XDG dirs

1. Recursive deletion is currently done for ~/.ghcup/ghc/<ver>/. We could instead record all files and have a proper deletion.
2. What to do with ~/.ghcup/bin files is another question, especially when XDG dirs are enabled (we don't have a guarantee we won't delete non-ghcup installed files if there are name clashes).

This could easily grow in complexity though, because we don't want to turn ghcup into a package manager.

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Wrt 2. we could also just avoid using ~/.local/bin and instead use ~/.local/share/ghcup/bin, because the former is not (yet) in the XDG standard: https://gitlab.freedesktop.org/xdg/xdg-specs/-/issues/63