Mark imports as implicit

[tfausak]

Consider a simple module like this:

{-# OPTIONS_GHC -Wunused-imports -fplugin=Imp #-}
main = System.IO.print ()

Before the changes in this PR, compiling that module would generate a warning like this:

<no location info>: warning: [GHC-66111] [-Wunused-imports]
    The qualified import of ‘System.IO’ is redundant
      except perhaps to import instances from ‘System.IO’
    To import instances alone, use: import System.IO()

That's not ideal since (a) the import is actually used and (b) the user didn't import the module themselves anyway, so they have no way to fix the warning. By changing it to an implicit import, the warning can be avoided.

Ideally there would be some other way to avoid this warning, but I don't know of any alternative.

[tfausak]

I didn't realize that implicit imports are output differently than normal ones. It's a bit weird that they're not even valid syntax, but I suppose that makes sense because they're supposed to be implicit anyway. It also makes the tests a little nicer because I can be sure the imports were actually inserted by Imp (rather than somehow coming from the input or parser).