Remove magic handling of FS_xxx symbols in exportRuntime. NFC (#21878)

This helps with my work towards #8380

src/library_fs.js

@@ -1863,4 +1863,7 @@ FS.staticInit();` +
    */`,
   $FS_mkdirTree__deps: ['$FS'],
   $FS_mkdirTree: (path, mode) => FS.mkdirTree(path, mode),
+
+  $FS_createLazyFile__deps: ['$FS'],
+  $FS_createLazyFile: 'FS.createLazyFile',
 });

src/library_fs_shared.js

@@ -170,6 +170,18 @@ addToLibrary({
     }
     return FS_stdin_getChar_buffer.shift();
   },
+
+  $FS_unlink__deps: ['$FS'],
+  $FS_unlink: 'FS.unlink',
+
+  $FS_createPath__deps: ['$FS'],
+  $FS_createPath: 'FS.createPath',
+
+  $FS_createDevice__deps: ['$FS'],
+  $FS_createDevice: 'FS.createDevice',
+
+  $FS_readFile__deps: ['$FS'],
+  $FS_readFile: 'FS.readFile',
 });
 
 // Normally only the FS things that the compiler sees are needed are included.

src/modules.mjs

@@ -371,23 +371,11 @@ function exportRuntime() {
   const EXPORTED_RUNTIME_METHODS_SET = new Set(EXPORTED_RUNTIME_METHODS);
 
   // optionally export something.
-  // in ASSERTIONS mode we show a useful error if it is used without
-  // being exported. how we show the message depends on whether it's
-  // a function (almost all of them) or a number.
   function maybeExport(name) {
-    // HEAP objects are exported separately in updateMemoryViews
-    if (name.startsWith('HEAP')) {
-      return;
-    }
-    // if requested to be exported, export it
-    if (EXPORTED_RUNTIME_METHODS_SET.has(name)) {
-      let exported = name;
-      // the exported name may differ from the internal name
-      if (exported.startsWith('FS_')) {
-        // this is a filesystem value, FS.x exported as FS_x
-        exported = 'FS.' + exported.substr(3);
-      }
-      return `Module['${name}'] = ${exported};`;
+    // If requested to be exported, export it.  HEAP objects are exported
+    // separately in updateMemoryViews
+    if (EXPORTED_RUNTIME_METHODS_SET.has(name) && !name.startsWith('HEAP')) {
+      return `Module['${name}'] = ${name};`;
     }
   }
 
@@ -401,12 +389,6 @@ function exportRuntime() {
     'addOnPostRun',
     'addRunDependency',
     'removeRunDependency',
-    'FS_createFolder',
-    'FS_createPath',
-    'FS_createLazyFile',
-    'FS_createLink',
-    'FS_createDevice',
-    'FS_readFile',
     'out',
     'err',
     'callMain',
@@ -500,6 +482,8 @@ function exportRuntime() {
   const results = exports.filter((name) => name);
 
   if (ASSERTIONS && !EXPORT_ALL) {
+    // in ASSERTIONS mode we show a useful error if it is used without being
+    // exported.  See `unexportedRuntimeSymbol` in runtime_debug.js.
     const unusedLibSymbols = getUnusedLibrarySymbols();
     if (unusedLibSymbols.size) {
       results.push(addMissingLibraryStubs(unusedLibSymbols));