Proper relative paths for script resolvers

[snoyberg]

Previously, any relative paths in a script's --resolver flag would be
considered relative to the current working directory. It's more logical
and consistent to have it relative to the directory the script lives in.
This makes the change.

Possible argument: it's a bit weird to put the resolver root into the
GlobalOptsMonoid value. Overall it seems like the best option to me, but
I'm open to other approaches.

Note: Documentation fixes for https://docs.haskellstack.org/en/stable/ should target the "stable" branch, not master.

Please include the following checklist in your PR:

• Any changes that could be relevant to users have been recorded in the ChangeLog.md
• The documentation has been updated, if necessary.

Please also shortly describe how you tested your change. Bonus points for added tests!