Replace use of whitelist/blacklist in \Drupal\Core\Extension classes

Problem/Motivation

Let's remove usage of "blacklist" and "whitelist" in \Drupal\Core\Extension classes and their tests

They are:

• An historic bad labelling of people
• Provide no context: "what is listed in them"?

Proposed resolution

• Fix Drupal\Core\Extension\Discovery\RecursiveExtensionFilterIterator:
  • Rename $blacklist to $skippedFolders
  • Rename $whitelist to $allowedExtensionTypes
• Fix comments in related code (core/lib/Drupal/Core/Extension/ExtensionDiscovery.php)

Remaining tasks

1. Finish fixing the patch (final points in #15)
2. Final reviews
3. RTBC
4. Commit
5. Publish CR

User interface changes

None

API changes

Not API, but we're renaming some protected array members in Drupal\Core\Extension\Discovery\RecursiveExtensionFilterIterator:

• $blacklist is replaced with $skippedFolders
• $whitelist is replaced with $allowedExtensionTypes

Data model changes

N/A

Release notes snippet

N/A