ESM library output support

[JSerFeng]

We've introduced EsmLibraryPlugin to make library output more clean and analyzable. But it's still experimental, here are some limits for now:

Runtime

Runtime code is still not good enough, we should avoid using the __webpack_require__ as the whole runtime namespace, every runtime method assigns itself to the __webpack_require__ object. This makes it hard for bundler to analyse useless runtime code and remove them.

__webpack_require__.r = ...
__webpack_require__.d = ...

Possible solution

Ideally, we should refactored each of them into pure util functions, and exports them.

Split chunks causing cycles

Well, yes, SplitChunks can work as expected for output code, but not as expected for running. SplitChunks for ESM output may not work as your expectation.

For example, imagine there is a module graph looks like this: A imports B imports C. We know if we do not split any module, there would be only 1 single chunk contains A, B and C. But if we split B into another chunk, if Rspack do nothing extra, the output are 2 chunks, the first chunk contains A and C, the second chunk contains B. A depends on B, so first chunk depends on second chunk, B depends on C, so second chunk depends on first chunk, there comes a cycle.

Possible solution

When we split a module, we either split the dependencies with it, or split its dependencies into another chunk. All we do is to make its dependencies away from the original chunk, so we can avoid the cycle.

But how we achieve it ? Modify the SplitChunksPlugin or we can make an easy one for library users ?