refactor(generators): Restructure generator modules to contain side effects (#7173)

* refactor(generators): Move lang.js -> lang/lang_gernator.js

  Move the LangGenerator definitions into their respective
  subdirectories and add a _generator suffix to their filenames,
  i.e. generators/javascript.js  becomes
  generators/javascript/javascript_generator.js.

  This is to keep related code together and allow the `lang/all.js`
  entrypoints to be moved to the top level generators/ directory.

  No goog module IDs were changed, so playground and test code
  that accesses this modules by filename does not need to be modified.

* refactor(generators) Move lang/all.js -> lang.js

  - Move the entrypoints in generators/*/all.js to correspondingly-named
    files in generators/ instead—i.e., generators/javascript/all.js
    becomes generators/javascript.js.

  - Update build_tasks.js accordingly.

* fix(generators): Add missing exports for LuaGenerator, PhpGenerator

  These were inadvertently omitted from #7161 and #7162, respectively.

* refactor(generators): Make block generator modules side-effect free

  - Move declaration of <lang>Generator instance from
    generators/<lang>/<lang>_generator.js to generators/<lang>.js.
  - Move .addReservedWords() calls from generators/<lang>/*.js to
    generators/<lang>.js
  - Modify generators/<lang>/*.js to export block generator functions
    individually, rather than installing on <lang>Generator instance.
  - Modify generators/<lang>.js to import and install block generator
    functions on <lang>Generator instance.

* fix(tests): Fix tests broken by restructuring of generators

  Where these tests needed block generator functions preinstalled
  they should have been importing the Blockly.<Lang>.all module.

  Where they do not need the provided block generator functions
  they can now create their own empty <Lang>Generator instances.

* chore: Update renamings file

  - Fix a malformation in previous entries that was not detected by
    the renaming file validator test.
  - Add entries describing the work done in this and related recent
    PRs.

* fix: Correct minor errors in PR #7173

  - Fix a search-and-replace error in renamings.json5
  - Fix an incorrect-but-usable import in generator_test.js

scripts/migration/renamings.json5

@@ -1376,6 +1376,8 @@
       oldName: 'Blockly.Python',
       newExport: 'pythonGenerator',
       newPath: 'Blockly.Python',
+    },
+    {
       oldName: 'Blockly.ContextMenuRegistry',
       exports: {
         'ContextMenuRegistry.ScopeType': {
@@ -1510,5 +1512,61 @@
         },
       },
     },
+    // The following renamings serve two purposes:
+    // - Record that the langGenerator instances have moved to a
+    //   different module (though this is not actually actioned by the
+    //   renaming script).
+    // - Record a change of path, applicable only to code that imports
+    //   generators via <script> tags.
+    {
+      oldName: 'Blockly.Dart',
+      exports: {
+        dartGenerator: {
+          newModule: 'Blockly.Dart.all',
+          oldPath: 'Blockly.Dart',
+          newPath: 'dart.dartGenerator',
+        },
+      },
+    },
+    {
+      oldName: 'Blockly.JavaScript',
+      exports: {
+        javascriptGenerator: {
+          newModule: 'Blockly.Javascript.all',
+          oldPath: 'Blockly.JavaScript',
+          newPath: 'javascript.javascriptGenerator',
+        },
+      },
+    },
+    {
+      oldName: 'Blockly.Lua',
+      exports: {
+        luaGenerator: {
+          newModule: 'Blockly.Lua.all',
+          oldPath: 'Blockly.Lua',
+          newPath: 'lua.luaGenerator',
+        },
+      },
+    },
+    {
+      oldName: 'Blockly.Php',
+      exports: {
+        phpGenerator: {
+          newModule: 'Blockly.PHP.all',
+          oldPath: 'Blockly.PHP',
+          newPath: 'php.phpGenerator',
+        },
+      },
+    },
+    {
+      oldName: 'Blockly.Python',
+      exports: {
+        pythonGenerator: {
+          newModule: 'Blockly.Python.all',
+          oldPath: 'Blockly.Python',
+          newPath: 'python.pythonGenerator',
+        },
+      },
+    },
   ],
 }