Remove useless empty js output for CSS module

[JSerFeng]

Currently there is always a corresponding js module in the output, even the css module exports nothing.

The root cause of that is, the dependency point to the css module is HarmonySideEffectsDependency, it has 2 effects:

1. Connects to referenced module
2. Start executing the referenced module, (use __webpack_require__)

For css module whose and exports type is object (not stylesheet or string), we use SideEffectDependency just want to connect it. We can use replace it as a special ImportCSSdependency.

The dependency doesn't render, so there is no __webpack_require__('src/index.css') in the output. And when CSS module generates, it will look for all incoming connections to know what sourceType it should generate to. Connection with ImportCSSdependency can be skipped, so it won't generate unnecessary js code in the output.

For those who use exports from the css module, eg.

import { anything } from './index.css'  <- ImportCSSDependency

anything; <- importSpecifierDependency

It has importSpecifierDependency, so the referenced css still need to generate js output just like now, we just change the HarmonySideEffectsDependency

There is also a plan B

We just do something in HarmonySideEffectsDependency.Template in CssModulesPlugin, make it renders nothing, and in CssGenerator.getTypes we skip checking SideEffectDependency incoming. This is easier to implement

[JSerFeng]

cc @ahabhgk