MultiCompiler parallelism and dependencies

[sokra]

fixes #9162

What kind of change does this PR introduce?
feature, bugfix

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no a butfix.
dependencies is now handled in watch mode, which changes behavior a bit

What needs to be documented once your changes are merged?

• The dependencies config option is largely undocumented
  • It is an array of names of other configs in the same array of configs. All dependencies need to be compiled before this config starts compiling.
  • In watch mode dependencies will invalidate the compiler when the dependency has changed and when a dependency is currently compiling or invalid this config will not compile until the dependency is done.
• There is a parallelism: number option on the configuration array now
  • It specifies the maximum of compilers that will compile in parallel.

module.exports = [
  { ... },
  { ... }
];
module.exports.parallelism = 1;

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[alexander-akait]

Fix for #9162?

[webpack-bot]

@sokra The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from azure (2 errors / 6 warnings) and appveyor (success) and fix these issues.

[webpack-bot]

I've created an issue to document this in webpack/webpack.js.org.

[sokra]

yes fixes #9162