builder: Allow building modules with no sources if buildsystem=simple

This makes a lot of sense to have for simple things like just directly
modifying the /app root.

builder/builder-manifest.c

@@ -1530,9 +1530,6 @@ builder_manifest_build_shell (BuilderManifest *self,
   if (found == NULL)
     return flatpak_fail (error, "Can't find module %s", modulename);
 
-  if (!builder_module_get_sources (found))
-    return flatpak_fail (error, "No sources for module %s", modulename);
-
   if (!builder_module_build (found, NULL, context, TRUE, error))
     return FALSE;
 
@@ -1566,7 +1563,7 @@ builder_manifest_build (BuilderManifest *self,
           return TRUE;
         }
 
-      if (!builder_module_get_sources (m))
+      if (!builder_module_should_build (m))
         {
           g_print ("Skipping module %s (no sources)\n", name);
           continue;

builder/builder-module.c

@@ -826,6 +826,19 @@ builder_module_is_enabled (BuilderModule *self,
   return TRUE;
 }
 
+gboolean
+builder_module_should_build (BuilderModule *self)
+{
+  if (self->sources != NULL)
+    return TRUE;
+
+  /* We allow building simple types even without sources, that is often useful */
+  if (!g_strcmp0 (self->buildsystem, "simple"))
+    return TRUE;
+
+  return FALSE;
+}
+
 gboolean
 builder_module_get_disabled (BuilderModule *self)
 {

builder/builder-module.h

@@ -43,6 +43,7 @@ const char * builder_module_get_name (BuilderModule *self);
 gboolean     builder_module_is_enabled (BuilderModule *self,
                                         BuilderContext *context);
 gboolean     builder_module_get_disabled (BuilderModule *self);
+gboolean     builder_module_should_build (BuilderModule *self);
 GList *      builder_module_get_sources (BuilderModule *self);
 GList *      builder_module_get_modules (BuilderModule *self);
 void         builder_module_set_json_path (BuilderModule *self,