Move the __non_webpack_import__ re-writing into the Babel plugin

Note how we're using custom `__non_webpack_import__`-calls in the code-base, that we replace during the post-processing stage of the build, to be able to write `import`-calls that Webpack will leave alone during parsing.
This work-around is necessary since we let Babel discards all comments, given that we generally don't need/want them in the builds, hence why we cannot utilize `/* webpackIgnore: true */`-comments in the source-code.

After the changes in PR 17563 it thus seems to me that we should be able to just move this re-writing into the Babel plugin instead.

external/builder/fixtures_esprima/importalias.js

@@ -1,3 +1,4 @@
 import { Test } from 'import-alias';
 import { Test2 } from './non-alias';
 export { Test3 } from 'import-alias';
+await __non_webpack_import__("./non-alias");