Fix disabled usermod presence validation

PlatformIO doesn't clean out the libdir when usermods are disabled, so
they still appear in the LibBuilders() set.  Ensure that we validate
only usermods that were actually deps for the build.

pio-scripts/validate_modules.py

@@ -53,9 +53,10 @@ def validate_map_file(source, target, env):
         secho(f"ERROR: Map file not found: {map_file_path}", fg="red", err=True)
         Exit(1)
 
-    # Identify the WLED module source directories
-    module_lib_builders = [builder for builder in env.GetLibBuilders() if is_wled_module(env, builder)]
+    # Identify the WLED module builders, set by load_usermods.py
+    module_lib_builders = env['WLED_MODULES']
 
+    # Filter/warn if an incompatible usermod was requested
     if env.GetProjectOption("custom_usermods","") == "*":
         # All usermods build; filter non-platform-OK modules
         module_lib_builders = [builder for builder in module_lib_builders if env.IsCompatibleLibBuilder(builder)]