Compatibility for 7.4 and 7.8

This address @23Skidoo's comment
https://github.com/haskell/cabal/pull/2500#issuecomment-8703532

cabal-install/cabal-install.cabal

@@ -203,6 +203,7 @@ Test-Suite unit-tests
         tasty,
         tasty-hunit,
         tasty-quickcheck,
+        tagged,
         QuickCheck >= 2.5
 
   if flag(old-directory)

cabal-install/tests/UnitTests/Distribution/Client/Dependency/Modular/Solver.hs

@@ -7,6 +7,7 @@ module UnitTests.Distribution.Client.Dependency.Modular.Solver (tests, options)
 import Control.Monad
 import Data.Maybe (catMaybes, isNothing)
 import Data.Either (partitionEithers)
+import Data.Proxy
 import Data.Typeable
 import Data.Version
 import qualified Data.Map as Map