Local versions

[hasufell]

Remarks:

1. The shim is hardcoded in ghcup currently. So if there is a bug, then we need to tell people to 'ghcup upgrade'. That could be a problem imo.
2. the shim does some additional work, I'm not sure if this will in fact slow down compilation processes? A cat call and a [ -f file ] check.
3. the local feature only works for the current directory (and only the current directory), there is no recursive upwards check performed, so if you are in a subdirectory of your project, it will not work

TODO:

• I think there was some logic checking for ghc etc being dangling symlinks, clean that up