Only mark _scriptDir as extern in MODULARIZE mode. NFC (#21734)

Outside of `MODULARIZE` this is just a normal variable that can/should
be minified.

Author: sbc100

src/closure-externs/closure-externs.js

@@ -196,14 +196,6 @@ var removeEventListener = function (type, listener) {};
  */
 var close;
 
-// Due to the way MODULARIZE works, Closure is run on generated code that does not define _scriptDir,
-// but only after MODULARIZE has finished, _scriptDir is injected to the generated code.
-// Therefore it cannot be minified.
-/**
- * @suppress {duplicate, undefinedVars}
- */
-var _scriptDir;
-
 // Closure run on asm.js uses a hack to execute only on shell code, declare externs needed for it.
 /**
  * @suppress {undefinedVars}

src/closure-externs/modularize-externs.js

@@ -0,0 +1,7 @@
+// Due to the way MODULARIZE works, Closure is run on generated code that does not define _scriptDir,
+// but only after MODULARIZE has finished, _scriptDir is injected to the generated code.
+// Therefore it cannot be minified.
+/**
+ * @suppress {duplicate, undefinedVars}
+ */
+var _scriptDir;

test/other/metadce/test_metadce_minimal_pthreads.gzsize

@@ -1 +1 @@
-6104
+6098

test/other/metadce/test_metadce_minimal_pthreads.jssize

@@ -1 +1 @@
-13481
+13448

tools/building.py

@@ -555,6 +555,9 @@ def closure_compiler(filename, advanced=True, extra_closure_args=None):
   # should not minify these symbol names.
   CLOSURE_EXTERNS = [path_from_root('src/closure-externs/closure-externs.js')]
 
+  if settings.MODULARIZE:
+    CLOSURE_EXTERNS += [path_from_root('src/closure-externs/modularize-externs.js')]
+
   if settings.USE_WEBGPU:
     CLOSURE_EXTERNS += [path_from_root('src/closure-externs/webgpu-externs.js')]