Only enable -Wunused-packages when ghc >=9

Cabal-syntax/Cabal-syntax.cabal

@@ -59,7 +59,9 @@ library
   if impl(ghc >= 8.0) && impl(ghc < 8.8)
     ghc-options: -Wnoncanonical-monadfail-instances
 
-  if impl(ghc >= 8.10)
+  -- Warning: even though introduced with GHC 8.10, -Wunused-packages gives
+  -- false positives with GHC 8.10.
+  if impl(ghc >= 9)
     ghc-options: -Wunused-packages
 
   build-tool-depends: alex:alex

Cabal/Cabal.cabal

@@ -65,7 +65,9 @@ library
   if impl(ghc >= 8.0) && impl(ghc < 8.8)
     ghc-options: -Wnoncanonical-monadfail-instances
 
-  if impl(ghc >= 8.10)
+  -- Warning: even though introduced with GHC 8.10, -Wunused-packages gives
+  -- false positives with GHC 8.10.
+  if impl(ghc >= 9)
     ghc-options: -Wunused-packages
 
   exposed-modules:

cabal-install-solver/cabal-install-solver.cabal

@@ -40,10 +40,12 @@ library
     -Wall -Wcompat -Wnoncanonical-monad-instances
     -fwarn-tabs -fwarn-incomplete-uni-patterns
 
-  if impl(ghc <8.8)
+  if impl(ghc < 8.8)
     ghc-options: -Wnoncanonical-monadfail-instances
 
-  if impl(ghc >=8.10)
+  -- Warning: even though introduced with GHC 8.10, -Wunused-packages gives
+  -- false positives with GHC 8.10.
+  if impl(ghc >= 9)
     ghc-options: -Wunused-packages
 
   exposed-modules:

cabal-install/cabal-install.cabal

@@ -51,9 +51,9 @@ common warnings
     if impl(ghc < 8.8)
       ghc-options: -Wnoncanonical-monadfail-instances
 
-    if impl(ghc >=9.0)
-      -- Warning: even though introduced with GHC 8.10, -Wunused-packages
-      -- gives false positives with GHC 8.10.
+    -- Warning: even though introduced with GHC 8.10, -Wunused-packages gives
+    -- false positives with GHC 8.10.
+    if impl(ghc >= 9)
       ghc-options: -Wunused-packages
 
 common base-dep

project-cabal/ghc-options.config

@@ -3,7 +3,9 @@ program-options
     -fno-ignore-asserts
     -Werror
 
-if impl(ghc >= 8.10)
+-- Warning: even though introduced with GHC 8.10, -Wunused-packages gives false
+-- positives with GHC 8.10.
+if impl(ghc >= 9)
   program-options
     ghc-options: -Wunused-packages
   package cabal-testsuite