init: Load snapshots on demand during resolver selection

This fixes a regression from 6047a31 that
caused _all_ the recommended snapshots to be loaded upfront.

Author: sjakobi

src/Stack/BuildPlan.hs

@@ -431,13 +431,13 @@ selectBestSnapshot
     :: (HasConfig env, HasGHCVariant env)
     => Path Abs Dir -- ^ project root, used for checking out necessary files
     -> [GenericPackageDescription]
-    -> NonEmpty SnapshotDef
+    -> NonEmpty SnapName
     -> RIO env (SnapshotDef, BuildPlanCheck)
 selectBestSnapshot root gpds snaps = do
     logInfo $ "Selecting the best among "
                <> T.pack (show (NonEmpty.length snaps))
                <> " snapshots...\n"
-    F.foldr1 go (NonEmpty.map getResult snaps)
+    F.foldr1 go (NonEmpty.map (getResult <=< loadResolver . ResolverSnapshot) snaps)
     where
         go mold mnew = do
             old@(_snap, bpc) <- mold

src/Stack/Init.hs

@@ -360,8 +360,7 @@ getDefaultResolver whichCmd stackYaml initOpts mresolver bundle = do
         selectSnapResolver = do
             let gpds = Map.elems (fmap snd bundle)
             snaps <- fmap getRecommendedSnapshots getSnapshots'
-            sds <- mapM (loadResolver . ResolverSnapshot) snaps
-            (s, r) <- selectBestSnapshot (parent stackYaml) gpds sds
+            (s, r) <- selectBestSnapshot (parent stackYaml) gpds snaps
             case r of
                 BuildPlanCheckFail {} | not (omitPackages initOpts)
                         -> throwM (NoMatchingSnapshot whichCmd snaps)