fix: run modern build in separate process

[yyx990803]

fix #3438, #3474

Fixes modern mode issues by really running the two builds in separate processes (thus avoiding any babel preset caching issues)

[yyx990803]

We need to update the test case (e.g. using async/await in main.js) to verify the content of the two builds.

[haoqunjiang]

Added in 805a4ce

[fracsi]

For our project this breaks the build process. We call vue-cli-service from a subfolder through package.json (from project1 folder: ../node_modules/.bin/vue-cli-service build --modern --verbose).
This way the the Service context is set to the project subfolder, for normal build this is good, but when modern mode kicks in, the current implementation tries to find vue-cli-service runnable with PluginApi (https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-service/lib/commands/build/index.js#L61) to run it as a subprocess, but fails to locate vue-cli-service.

We use the same vue deps for different projects, so the whole build and dependency graph is the same.

Our project layout:

/node_modules
/project_1
    src/...
    package.json - "build": "../node_modules/.bin/vue-cli-service build --modern"
/project_2
    src/...
    package.json - "build": "../node_modules/.bin/vue-cli-service build --modern"
package.json - with vue dependecies

Project package.json

{
  "name": "project_1",
  "private": true,
  "scripts": {
    "serve": "../node_modules/.bin/vue-cli-service serve --verbose",
    "build": "../node_modules/.bin/vue-cli-service build --modern --verbose",
    "lint": "../node_modules/.bin/vue-cli-service lint"
  },
  "vuePlugins": {
    "resolveFrom": "../"
  }
}
```