[WIP] Add --json solver config arg

This allows the cabal-install solver logs to be mechanized, e.g., similar to
the tree view provided by nix-output-monitor for Nix.

cabal-install/src/Distribution/Client/Dependency.hs

@@ -59,6 +59,7 @@ module Distribution.Client.Dependency
   , setSolveExecutables
   , setGoalOrder
   , setSolverVerbosity
+  , setSolverOutputJson
   , removeLowerBounds
   , removeUpperBounds
   , addDefaultSetupDependencies
@@ -266,6 +267,7 @@ data DepResolverParams = DepResolverParams
   , depResolverGoalOrder :: Maybe (Variable QPN -> Variable QPN -> Ordering)
   -- ^ Function to override the solver's goal-ordering heuristics.
   , depResolverVerbosity :: Verbosity
+  , depResolverOutputJson :: Bool
   }
 
 showDepResolverParams :: DepResolverParams -> String
@@ -363,6 +365,7 @@ basicDepResolverParams installedPkgIndex sourcePkgIndex =
     , depResolverSolveExecutables = SolveExecutables True
     , depResolverGoalOrder = Nothing
     , depResolverVerbosity = normal
+    , depResolverOutputJson = False
     }
 
 addTargets
@@ -498,6 +501,12 @@ setSolverVerbosity verbosity params =
     { depResolverVerbosity = verbosity
     }
 
+setSolverOutputJson :: Bool -> DepResolverParams -> DepResolverParams
+setSolverOutputJson outputJson params =
+  params
+    { depResolverOutputJson = outputJson
+    }
+
 -- | Some packages are specific to a given compiler version and should never be
 -- reinstalled.
 dontInstallNonReinstallablePackages :: DepResolverParams -> DepResolverParams
@@ -886,13 +895,16 @@ resolveDependencies platform comp pkgConfigDB params =
                     solveExes
                     order
                     verbosity
+                    outputJson
                   ) =
         if asBool (depResolverAllowBootLibInstalls params)
           then params
           else dontInstallNonReinstallablePackages params
 
     formatProgress :: Progress SummarizedMessage String a -> Progress String String a
-    formatProgress p = foldProgress (\x xs -> Step (renderSummarizedMessage x) xs) Fail Done p
+    formatProgress p = foldProgress (\x xs -> Step (formatter x) xs) Fail Done p
+      where
+        formatter = if outputJson then error "FIXME: To be implemented ..." else renderSummarizedMessage
 
     preferences :: PackageName -> PackagePreferences
     preferences = interpretPackagesPreference targets defpref prefs
@@ -1210,6 +1222,7 @@ resolveWithoutDependencies
       _onlyConstrained
       _order
       _verbosity
+      _outputJson
     ) =
     collectEithers $ map selectPackage (Set.toList targets)
     where