fix(external_module): use "const" if supported when concatenation

[fi3ework]

Summary

Align with https://github.com/webpack/webpack/blob/039e8c96338ee98cf37d5035cdd30073a96b4ec5/lib/ExternalModule.js#L878. No coveraged test found in webpack.

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[JSerFeng]

Note: We can safely use const or let here, because external module has definite runtime condition, so the require won't be wrapped in if statement, which is like

if ("main" == __webpack_require__.j) {
  var foo = require('./foo')
}

So its safe if don't have hoisting here

So LGTM