Add tests for duplicate album imports
Test import singleton duplicates
Refactor ImportDuplicateAlbumTest
Remove DuplicateCheckTest

Superseded by `ImportDuplicate*Test`.
Add SingletonImportTask

The goal of this class is to eliminate all checks for `task.is_album`. This is
already accomplished for the methods. Next, we want to tackle the plugin
stages.
Add task.apply_metadata() method
Album tasks always record their progress
Add SentinelImportTask and move code into it
Make _resume() session local and refactor resume in read stage

Functionaly, this should not change anything.
Refactor finalize stage into Task class
Refactor task.finalize()
Merge singleton and album lookup stages
Merge duplicate detection into find_duplicates method
Add resolve_duplicates stage
Make _infer_album_fields a method