Register PurgeCSS content array as PostCSS dependencies

[bradlc]

This pull requests ensures that files and directories in the PurgeCSS content array are registered as PostCSS dependencies, so that runner caches are correctly invalidated when these change.

[foxted]

Hi there!

For some obscure reason, when using tailwindcss with Nuxt.js, affects Nuxt.js itself.

With tailwindcss@2.1.2, Nuxt.js was still behaving properly, watching only the files needed for the project (vue files, javascript and css).

After upgrading to tailwindcss@2.1.3, now Nuxt.js starts watching every single file in the project (including node modules, markdown files, IDE specific files (.idea/.vscode) etc..).

Not really sure how yet, I'll keep digging though.

EDIT: This new behaviour caused an issue with @nuxtjs/tailwindcss, because they use nuxt.config.{js,ts}, which is flagged as a glob, thus watching the entire directory where it is contained.

nuxt-modules/tailwindcss#359

[AlexanderBelokon]

I've created a minimal reproduction repo for this issue: https://github.com/AlexanderBelokon/nuxt-tailwind-watch