Avoid unnecessary code for unused lazy imports

[nicolo-ribaudo]

Q	A
Fixed Issues?
Patch: Bug Fix?
Major: Breaking Change?
Minor: New Feature?
Tests Added + Pass?	Yes
Documentation PR Link
Any Dependency Changes?
License	MIT

Thanks to #15446, I noticed that out released code include this:

function _module() {
  const data = require("module");
  _module = function () {
    return data;
  };
  return data;
}

even if that _module() function is then not used anywhere.

Babel generates that code when there is a import Module from "module" import, and it's then unused because all the usages of Module are stripped away at build time. With this PR, we avoid injecting any code for lazy imports that are not actually used, since it's code that would never be executed at runtime anyway.

As before, we continue emitting eager imports for import "x" since these import statements are explicitly used for side effects.

I have a feeling that #15446 is caused by trying to bundle @babel/core with a webpack version that throws by default when trying to include some node-specific modules. That issue is not a bug with Babel, but once we merge this PR and we release a Babel version compiled with this PR (so in at least two releases), that issue will be fixed because the dead require call will be removed.

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/54003/