Add PostCSS dependency message support

[ai]

🙋 feature request

PostCSS ecosystem has an unofficial API: plugins add dependency message for every resource used by this plugin:

let css = `
.icon {
  background: inline-svg(./logo.svg)
}
`

const result = await postcss([inlineSVG]).process(css)
result.messages() // => [{ type: 'dependency', file: '/Dev/project/logo.svg' }]

postcss-loader and other PostCSS runners use this message to start watching for used resources.

Let’s add it to Parcel too.

🤔 Expected Behavior

Parcel will watch for resources from dependency messages and re-build bundle on resource’s changes.

😯 Current Behavior

Seems like Parcel doesn’t use it.

💁 Possible Solution

Just check result.messages().filter(i => i.type === 'dependency') after PostCSS call.

🔦 Context

Lack of this feature was found in Tailwind community since the lack of dependency support creates unexpected behavior:

https://twitter.com/kevdotbadger/status/1125415276097544193

Here is our small talk with @adamwathan

https://twitter.com/adamwathan/status/1125417346154106881

💻 Examples

Here is a small project to test the feature:

https://github.com/adamwathan/parcel-postcss-dependency-message

[ai]

@devongovett if you like this feature, but have no time to implement it, I can ask my friends to help.

[mischnic]

Parcel will watch for resources from dependency messages and re-build bundle on resource’s changes.

This solves an issue supporting postcss import plugins would have #1165 (comment)