Cannot import message files with filename that contain hyphens in v11

[devilhyt]

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Description

An error occurs when importing message files with filenames that contain hyphens (en-gb, zh-hant, etc).

In v11, a script for building message files was updated (#8091). The code below reveals that we directly use the filenames from msg/json/*.json to name the default export object.
https://github.com/google/blockly/blob/9519333c28662d20a4f4b73a68641a74965187b5/scripts/gulpfiles/build_tasks.js#L658-L688
However, hyphens are not legal in javascript names, which seems to be the root cause of the issue.

Reproduction steps

1. Get Blockly through Create-package script.
2. Import message files in index.js (ex: en-gb).
   import * as enGb from 'blockly/msg/en-gb';

Stack trace

ERROR in ./node_modules/blockly/msg/en-gb.mjs 1:9
Module parse failed: Unexpected token (1:9)
You may need an appropriate loader to handle this file type, currently no loaders are configured to process this file. See https://webpack.js.org/concepts#loaders
> import en-gb from './en-gb.js';
| export const {
|   LOGIC_HUE,
 @ ./src/index.js 8:0-42 16:18-22

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