fix(build): Reintroduce shims for submodule entrypoints

This is in effect a partial rollback of PR google#7822.

This should solve issues encountered by users of bunders that don't
support exports at all (e.g. browserify) as well as ones that don't
support it in certain circumstances (e.g., when using webpack's
resolve.alias configuration option to alias 'blockly' to
'node_modules/blockly', as we formerly did in most plugins, which
causes webpack to ignore blockly's package.json entirely).

Assumptions:
- Such bundlers will _completely_ ignore the exports declaration.
- The bundles are intended to be used in a browser—or at least not
  in node.js—so the core entrypoint never needs to route to
  core-node.js.  This is reasonable since there's little reason to
  bundle code for node.js, and node.js has supported the exports
  clause since at least v12, consideably older than any version of
  node.js we officially support.
- It suffices to provide only a CJS entrypoint (because we can only
  provide CJS or ESM, not both.  (We could in future switch to
  providing only an ESM entrypoint instead, though.)

scripts/gulpfiles/package_tasks.js

@@ -111,6 +111,50 @@ function packageUMDBundle() {
       .pipe(gulp.dest(`${RELEASE_DIR}`));
 };
 
+
+/**
+ * This task creates shims for the submodule entrypoints, for the
+ * benefit of bundlers and other build tools that do not correctly
+ * support the exports declaration in package.json.  These shims just
+ * require() and reexport the corresponding *_compressed.js bundle.
+ *
+ * This should solve issues encountered by users of bundlers that don't
+ * support exports at all (e.g. browserify) as well as ones that don't
+ * support it in certain circumstances (e.g., when using webpack's
+ * resolve.alias configuration option to alias 'blockly' to
+ * 'node_modules/blockly', as we formerly did in most plugins, which
+ * causes webpack to ignore blockly's package.json entirely).
+ *
+ * Assumptions:
+ * - Such bundlers will _completely_ ignore the exports declaration.
+ * - The bundles are intended to be used in a browser—or at least not
+ *   in node.js—so the core entrypoint never needs to route to
+ *   core-node.js.  This is reasonable since there's little reason to
+ *   bundle code for node.js, and node.js has supported the exports
+ *   clause since at least v12, consideably older than any version of
+ *   node.js we officially support.
+ * - It suffices to provide only a CJS entrypoint (because we can only
+ *   provide CJS or ESM, not both.  (We could in future switch to
+ *   providing only an ESM entrypoint instead, though.)
+ *
+ * @param {Function} done Callback to call when done.
+ */
+function packageLegacyEntrypoints(done) {
+  for (entrypoint of [
+    'core', 'blocks', 'dart', 'javascript', 'lua', 'php', 'python'
+  ]) {
+    const bundle =
+        (entrypoint === 'core' ? 'blockly' : entrypoint) + '_compressed.js';
+    fs.writeFileSync(path.join(RELEASE_DIR, `${entrypoint}.js`),
+        `// Shim for backwards-compatibility with bundlers that do not
+// support the 'exports' clause in package.json, to allow them
+// to load the blockly/${entrypoint} submodule entrypoint.
+module.exports = require('./${bundle}');
+`);
+  }
+  done();
+}
+
 /**
  * This task copies all the media/* files into the release directory.
  */
@@ -126,8 +170,10 @@ function packageMedia() {
  * - The scripts section is removed.
  *
  * Prerequisite: buildLangfiles.
+ *
+ * @param {Function} done Callback to call when done.
  */
-function packageJSON(cb) {
+function packageJSON(done) {
   // Copy package.json, so we can safely modify it.
   const json = JSON.parse(JSON.stringify(getPackageJson()));
   // Remove unwanted entries.
@@ -138,7 +184,7 @@ function packageJSON(cb) {
   }
   fs.writeFileSync(`${RELEASE_DIR}/package.json`,
       JSON.stringify(json, null, 2));
-  cb();
+  done();
 };
 
 /**
@@ -195,6 +241,7 @@ const package = gulp.series(
     gulp.parallel(
         packageIndex,
         packageCoreNode,
+        packageLegacyEntrypoints,
         packageMedia,
         gulp.series(packageLocales, packageUMDBundle),
         packageJSON,