Add "required" components for a given feature

Problem/Motivation

It will sometimes be necessary to override/customize the automatic assignment that plugins do, by explicitly adding an item to a given feature. Such overrides should persist when features are regenerated. This would be equivalent to the parallel case that's already implemented: the 'excluded' array. See work completed in #2595245: "excluded" configuration should not be automatically packaged.

This is a blocker for #2490888: 'existing' assignment plugin runs too early.

Proposed resolution

In the 'packages' assignment plugin, iterate through features info array and test for $info['features']['required']. If not empty, iterate and assign required items to the feature.

Add handling to the feature edit form parallel to the existing method for excluding components from a given feature. In the code base, 'excluded' and 'required' are referred to as constraints. Handle the interaction between these to flags in a toggling way, such that only one of the two flags can be set at a given time for a given item and toggling the item's selected status either removes any existing constraint or adds the appropriate constraint. Specifically:

• When an item that was previously unselected is selected:
  • If it was previously excluded, set it as no long excluded.
  • Otherwise, set it as required.
• When an item that was previously selected is unselected:
  • If it was previously required, set it as no long required.
  • Otherwise, set it as excluded.

Remaining tasks

User interface changes

API changes