Avoid clearing the `comp` flag on non-autotagged imports

In beets/importer.py an `align_album_level_fields` method tries to guess values for albumartist and comp attributes. But this do not work in some cases. For instance, if I import only one file from a compilation album, it will think it is a single artist album, and the path where to move the file will be wrong.
I commented out the `changes` in my local version of beet (which is very nice, by the way) so I can continue working, but to me it is better to not overwrite tags by guessing.

So maybe comp can be kept like this, but then add a compilation tag which has the real original value.

(Note about what I'm trying to do, which might be unusual: I have a few hundres of GB of media files, most properly tagged and organized qith Picard. I'm trying to replace picard with beet, and first step is to import my full media lib without writing any tags, and without changing the layout)

Start in beets/importer.py at align_album_level_fields and trace how a single-file import from a compilation album determines albumartist, comp, and the destination path. Reproduce the described import case without writing tags, then establish the expected preservation of existing values and add regression coverage for the corrected behavior.

Written by the indexing model from the issue text.