In duplicate resolution for singletons, show whether the duplicate track is part of an album

### Problem

My `beet` workflow is usually the same when importing:
* import albums first
* deal with singles after that.

When running `beet import -s `, `beet` may find duplicates (ie. tracks already in my library) and prompt for what to do.

The problem is that duplicates only show the user size/quality/duration of the existing file, but not if it is a single or an album track. This is dangerous IMHO because if the new single has a better quality, the user would tend to remove the album track from his/her library (therefore leading to an incomplete album).

Maybe these changes would help mitigate the problem:
* print the album name when a duplicate is found
* when importing singles, only consider singles as duplicate candidates

### Setup

* OS: Linux Debian Stretch
* beets version 1.4.3
* Python version 2.7.13
* plugins: chroma, discogs, embedart, fetchart, fromfilename, info, inline, lastgenre, replaygain, scrub
* Turning off plugins made problem go away (yes/no): no

Start at the `beet import -s ` flow where duplicate candidates are selected and the prompt displays existing-file details. Trace how album and single metadata are available there, then confirm the intended behavior with maintainers because the issue offers two possible changes. Done should make duplicate resolution distinguish album tracks from singles without risking incomplete albums.

Written by the indexing model from the issue text.