Re-import detection should permit partial and multiple overlap

_Re-imports_ occur when the user wants to re-tag some files that are already in their library. Currently, an album is considered a re-import only when it has exactly the same set of paths as an existing album. We should also permit re-imports that:
- Partially overlap with an existing album. This happens when the user wants to add tracks to an existing album, as in #112.
- Consist of multiple albums. This happens when the user erroneously imported a multi-disc album as separate albums initially but wants to re-import them together (e.g., with `--flat`).

The point where this is detected is in `beets.importer._duplicate_check`. We also need to support the appropriate replacement action in `apply_choices`.

This is especially pernicious since the user sees a "this album is already in the library!" prompt when the re-import check fails. If they choose "remove old", then some of the files being imported are deleted. No good!