Give warning on users attempting to prefix `purescript-`

[justinwoo]

e.g. psc-package install purescript-aff should warn

[paf31]

Now I can't release my purescript-purescript-parser library without users getting a warning! :)

[justinwoo]

I know right?? I was thinking that it'd be a final check if no package was found, not eager

[paf31]

Gotcha. I'll retract my 👎 then.

[justinwoo]

Fixed by #125