Allow plugins to override other plugins' options

[innocenzi]

As a follow-up of #6779, I'm opening this discussion to find a generic solution.

It would be super useful for plugins to be able to hook into the configuration of other plugins. At least that is the initial problem, but I think some sort of API that plugin authors can use to retrieve a value that could possibly have been modified by other plugins would be great.

For example, given this scenario:

• pluginA: offers a foo option in its factory function
• pluginB: depends on pluginA and needs foo to be set to overriden-foo-value

Vite could offer an API for pluginB to inform pluginA that foo's value needs to be overriden-foo-value. The main use case for me would be to allow back-end plugins to configure other plugins like Vue. A specific example is how Laravel must inform its users to configure Vue themselves: https://laravel.com/docs/9.x/vite#vue

I could imagine the API being something like this:

// pluginA
export default function pluginA(options?: Options) {
  options = {
    foo: 'default-foo-value',
    ...options
  }

  return {
    name: 'pluginA',
    configResolved: ({ pluginOverrides }) => {
      // Here `pluginOverrides` could contain the overrides defined by some other plugin
      // and pluginA could chose how to use it
      options.foo = pluginOverrides.pluginA.foo ?? options.foo

      // options = { ...options, ...pluginOverrides.pluginA }
    },
    // ...
  }
}

// pluginB
export default function pluginB() {
  return {
    name: 'pluginB',
    overridePlugins: () => ({
      pluginA: {
        foo: 'overriden-foo-value'
      }
    })
  }
}

Vite could call overridePlugins on all registered plugins before anything, so pluginOverrides could be filled in other hooks.

That's just one potential idea, but you get the gist of the initial problem - if you can think of something better, feel free to suggest!