fix: some package's module entry doesn't have extension

[FallenMax]

Some popular libs(e.g. rc-tooltip) are setting 'package.json#module' field WITHOUT file extension (.js). For better compatibility, we should add for them?

[yyx990803]

Thanks for the PR - but we need to

1. Only append extension if there isn't one (some packages may use non-js entry)
2. Also append the extension to the resolved entryPoint, not just the file path.

See 6269b7f