Validate the CSS categories in libraries.yml files.

Problem/Motivation

Let's add a new library definition:

drupal.node:
  version: VERSION
  css:
    css/node.module.css: {}

... this looks reasonable, but in fact this is broken. You need to specify a category:

drupal.node:
  version: VERSION
  css:
    layout:
      css/node.module.css: {}

This is far from being obvious.

Beta phase evaluation

Proposed resolution

Add some validation code in LibraryParser.php to ensure the "category" is right.

In case you use a path as category, throw an exception like:

You have to key the css files in the "$module.library.yml" file by category (css/node.module.css seems is a filename, allowed categories are ...)

In case someone actually uses a category, throw an exception like:

You use an invalid category "invalid_cateogry" in "module.libraries.yml". Allowed categories are ...

An example for such a library.yml file would be:

drupal.node:
  version: VERSION
  css:
    invalid_category:
      css/node.module.css: {}

Remaining tasks

Fix tests that are not using categories.

User interface changes

API changes