Improve outdated ghc-pkg cache warning
Sometimes you get this:
WARNING: cache is out of date: /home/user/.ghc/x86_64-linux-7.8.3/package.conf.d/package.cache. Use 'ghc-pkg recache' to fix
The warning is correct but a bit confusing, because running
ghc-pkg recache does nothing here: It will by default update your global package DB, and the DB the warning is about will still be out of date.
This seems to have frustrated others as well: https://gist.github.com/mcandre/5789498
I suggest we make the message more clear:
You might have to pass --user to update this cache file.