Test against init.el with all module flags enabled

Detect module flags by walking package.el files.

This still does not build all dependencies, because some are enabled
only if some flag or other module is disabled. But this should be close.

default.nix

@@ -71,6 +71,22 @@ let
     ${runtimeShell} ${doomSource}/bin/doomscript ${./build-helpers/full-init} -o $out
   '';
 
+  doomDirWithAllModulesAndFlags = runCommandLocal "doom-full-init"
+    {
+      env = {
+        EMACS = lib.getExe emacs;
+        # Enable this to troubleshoot failures at this step.
+        #DEBUG = "1";
+      };
+      # We set DOOMLOCALDIR somewhere harmless below to stop Doom from trying to
+      # create it somewhere read-only.
+    } ''
+    mkdir $out
+    export DOOMLOCALDIR=$(mktemp -d)
+    ${runtimeShell} ${doomSource}/bin/doomscript ${./build-helpers/full-init} --flags -o $out
+  '';
+
+
   # Step 1: determine which Emacs packages to pull in.
   #
   # Inputs: Doom, original DOOMDIR (only init.el and packages.el are used).
@@ -426,5 +442,5 @@ let
   '';
 in
 {
-  inherit doomDirWithAllModules doomEmacs emacsWithDoom;
+  inherit doomDirWithAllModules doomDirWithAllModulesAndFlags doomEmacs emacsWithDoom;
 }