feat: support multiple css-modules imports

[escaton]

Currently vue-loader doesn't support multiple css module sections.
In this example the last one will rewrite the $style object, and $style.root will be undefined.

<template>
  <div :class="$style.root">
    <div :class="$style.child />
  </div>
</template>

<style module critical>
.root { background: black; }
</style>

<style module>
.child { background: red; }
</style>

This PR fixes that.

[escaton]

Found a duplicate PR: #2045 with slightly different approach to HMR, but a lot more tests

[escaton]

@yyx990803 @Jinjiang calling you directly, as the repo seems almost abandoned. Could you please take a look at this or #2045 and merge one of them?