Refactor ReviewerConfig and ReviewerFilterSection

* Extract abstract class ReviewerFilter.
  - Abstracts the Config from REST and reviewer-adding logic in
  - Makes it more straight-forward to create ReviewerFilters in tests
    for comparison.
* Introduce ReviewerFilterCollection class.
  - Maintains the collection of ReviewerFilters found in the Config
    object provided to the ReviewerFilterCollection.
  - Handles adding and removing reviewers from a Config object through
    it's own implementation of ReviewerFilter.
* Create new config package.
  - Move ReviewerFilterCollection, ReviewersConfig and
    ReviewersConfigIT into this new package for separation.
* Remove ReviewerConfig#forProject(Project.NameKey) creator.
  - This created a ForProject with a Config "with inheritance" to
    enable getting all ReviewerFilterSections with inheritance
    from a ForProject.
    Since ForProject is also a VersionedMetaData this Config was later
    replaced if the ForProject was loaded. This meant that ForProjects
    had different responsibilities and behavior depending on if they
    were loaded or not.
  - Replace this creator with new method:
    with the sole responsibility of returning all inherited

In preparation of supporting CC.

Feature: Issue 12903
Change-Id: Ie0593297bd5e52749993966458aa13dd42f28bfd
10 files changed