Import single-file directories "as tracks" by default.

### Enhancement

I have lots of artists with just one songs in my library and I am getting tired of pressing "t" during import. I'd like to be able to tell beets to import them "as tracks" by default. A configuration option to do this would by super helpful.

The issue does not name files, tests, or entry points. Start by locating the import path for single-file directories and the configuration definitions; done means a configuration option imports those directories as tracks by default without pressing “t”.

Written by the indexing model from the issue text.