Hadrian: build warnings (redundant constraints, unused imports, type operators)

Building Hadrian produces the following warnings:

src/Hadrian/Expression.hs:38:10: warning: [-Wredundant-constraints]
    • Redundant constraint: Semigroup a
    • In the instance declaration for ‘Monoid (Expr c b a)’
   |
38 | instance (Semigroup a, Monoid a) => Monoid (Expr c b a) where
   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

src/Hadrian/Expression.hs:84:13: warning: [-Wtype-equality-requires-operators]
    The use of ‘~’ without TypeOperators
    will become an error in a future GHC release.
    Suggested fix: Perhaps you intended to use TypeOperators
   |
84 | instance (c ~ c', b ~ b', ToPredicate p c' b') => ToPredicate (Expr c b p) c' b' where
   |             ^

src/Hadrian/Expression.hs:84:21: warning: [-Wtype-equality-requires-operators]
    The use of ‘~’ without TypeOperators
    will become an error in a future GHC release.
    Suggested fix: Perhaps you intended to use TypeOperators
   |
84 | instance (c ~ c', b ~ b', ToPredicate p c' b') => ToPredicate (Expr c b p) c' b' where
   |                     ^

src/Hadrian/Haskell/Cabal/Parse.hs:67:1: warning: [-Wunused-imports]
    The import of ‘Packages’ is redundant
      except perhaps to import instances from ‘Packages’
    To import instances alone, use: import Packages()
   |
67 | import Packages
   | ^^^^^^^^^^^^^^^