Test recomposing source with submodules

oddly this doesn't reproduce the problem I'm experiencing IRL where
"dirtying" the submodule has no effect until the cache is either nuked
or runs out of TTL

tests/flakes-with-submodules.sh

@@ -20,14 +20,9 @@ for repo in $flakeDir $nonFlakeDir $flakeWithSubmodules; do
     git -C $repo init
     git -C $repo config user.email "foobar@example.com"
     git -C $repo config user.name "Foobar"
-done
-
-for dir in $nonFlakeDir $flakeDir; do
-cat > $dir/README.md <<EOF
-FNORD
-EOF
-git -C $dir add README.md
-git -C $dir commit -m 'Initial'
+    echo FNORD > $repo/README.md
+    git -C $repo add README.md
+    git -C $repo commit -m 'Initial'
 done
 
 cp config.nix $flakeDir
@@ -81,6 +76,11 @@ cat > $flakeWithSubmodules/flake.nix <<EOF
         nonFlake = builtins.fetchTree (traceVal self.modules.nonFlake);
         flake = builtins.getFlake (traceVal self.modules.flake);
 
+        mapAttrsToList = f: attrs:
+          map (name: f name attrs.\${name}) (builtins.attrNames attrs);
+        pipe = val: functions:
+          let reverseApply = x: f: f x;
+          in builtins.foldl' reverseApply val functions;
         writeShellScriptBin = name: script: mkDerivation {
             inherit name;
             buildCommand = ''
@@ -91,12 +91,35 @@ cat > $flakeWithSubmodules/flake.nix <<EOF
                 chmod +x \$bin
             '';
         };
+
+        combineModules = self: let
+          srcs = builtins.mapAttrs (_: url: fetchTree (traceVal url)) self.modules;
+        in mkDerivation {
+          name = "source-with-submodules";
+          buildCommand = ''
+            cp -r \${self} \$out
+            chmod -R +w \$_
+
+            \${
+              pipe srcs [
+                (mapAttrsToList (path: src: "cp -rT \${src} \$out/\${path}"))
+                (builtins.concatStringsSep "\n")
+              ]
+            }
+          '';
+        };
     in {
         packages.$system = {
             cat-submodule-readme = writeShellScriptBin "cat-submodule-readme" ''
                 cat \${nonFlake}/README.md
             '';
             use-submodule-as-flake = flake.packages.$system.cat-own-readme;
+
+            cat-own-readme = writeShellScriptBin "cat-own-readme" ''
+                cat \${self}/README.md
+            '';
+
+            source-with-submodules = combineModules self;
         };
 
         defaultPackage.$system = self.packages.$system.cat-submodule-readme;
@@ -107,12 +130,19 @@ EOF
 git -C $flakeWithSubmodules add .
 git -C $flakeWithSubmodules commit -m'add flake.nix'
 
+[[ $(nix run $flakeWithSubmodules#cat-own-readme) == "FNORD" ]]
 [[ $(nix run $flakeWithSubmodules#cat-submodule-readme) == "FNORD" ]]
 [[ $(nix run $flakeWithSubmodules#use-submodule-as-flake) == "FNORD" ]]
+nix build -o $TEST_ROOT/result $flakeWithSubmodules#source-with-submodules
+[[ $(cat $TEST_ROOT/result/nonFlake/README.md) == "FNORD" ]]
+
+echo FOST > $flakeWithSubmodules/README.md
 
 # apply dirt
 echo FSUD > $flakeWithSubmodules/nonFlake/README.md
 [[ $(nix run $flakeWithSubmodules#cat-submodule-readme) == "FSUD" ]]
+nix build -o $TEST_ROOT/result $flakeWithSubmodules#source-with-submodules
+[[ $(cat $TEST_ROOT/result/nonFlake/README.md) == "FSUD" ]]
 
 # should work for flake as well
 echo FSUD > $flakeWithSubmodules/flake/README.md