refactor(d9): readded core: 8.x. Switched AliasManager to AliasManagerInterface
Closes #3227011
Added some additional changes based on my colleagues suggestions:
- Readded core: 8.x to the .info file since Drupal Core versions before 8.7.7 do not recognise the core_version_requirement: key.
- Switched the AliasManager Class with AliasManagerInterface. This is just in cause AliasManager is replaced with another implementation