fix: only render init code when need moduleRemapping

[fi3ework]

What kind of change does this PR introduce?

For ExternalModule, render init code only when needing moduleRemapping, which will reduce useless code. Although the init code could be removed when minimizing, but this will help in some scenario, for example, we are using webpack to building bundleless package without running minimizing, this will help.

Did you add tests for your changes?

No, the test code is not minimized, they should sufficient so far. Feel free to let me what kind of case should be added. :)

Does this PR introduce a breaking change?

No.

What needs to be documented once your changes are merged?

No.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[alexander-akait]

Thank you