cabal v2-install fails late if symlink target already exists

[fgaz]

(continuing from #5491)

Right now cabal v2-install x first builds x and then attempts to symlink its executables, failing if they already exist.

Instead, we should check for existing symlinks before building anything, and fail as early as possible. See https://github.com/haskell/cabal/issues/5491#issuecomment-433032100 and previous comments

[jneira]

@fgaz hi, have you still plans to work on this? it seems a not too hard a nice change so i am gonna label with pr welcome