Re-enable upgrade functionality for all configurations

Adds a --fail-if-shadowed switch.

ghcup.cabal

@@ -48,13 +48,6 @@ flag no-exe
   default:     False
   manual:      True
 
-flag disable-upgrade
-  description:
-    Disable upgrade functionality. This is mainly to support brew packagers.
-
-  default:     False
-  manual:      True
-
 library
   exposed-modules:
     GHCup
@@ -204,6 +197,7 @@ executable ghcup
     GHCup.OptParse.Set
     GHCup.OptParse.ToolRequirements
     GHCup.OptParse.UnSet
+    GHCup.OptParse.Upgrade
     GHCup.OptParse.Whereis
 
   hs-source-dirs:     app/ghcup
@@ -277,11 +271,6 @@ executable ghcup
   if flag(no-exe)
     buildable: False
 
-  if flag(disable-upgrade)
-    cpp-options: -DDISABLE_UPGRADE
-
-  else
-    other-modules: GHCup.OptParse.Upgrade
 
 test-suite ghcup-test
   type:               exitcode-stdio-1.0