Language.Haskell.TH.Lib.Internal type synonyms leak into error messages

Error messages from uses of TH quotes will feature type synonyms from the Lib.Internal module.

For instance:

 • Couldn't match type ‘[Dec]’ with ‘Dec’                                                                                
      Expected: Q Dec                                                                                                       
        Actual: Q Language.Haskell.TH.Lib.Internal.Decs                                                                     

This Decs type synonym just stands for [Dec]. It's a bit confusing to point users to this internal module.

We should refactor how desugaring of TH quotes works to avoid referring to these names in the code we generate.