Handle multiple licenses in the /licenses folder

https://reuse.software/practices/2.0/ recommends a format for including multiple licenses in a repo/component (use a `LICENSES` folder). We should include this in the summarization as sources of `declared.license`. Where there are multiple, these should be AND'd together.

Note that Licensee, Scancode, ... should find the files as full licesne text (nature = license) independent of any naming or location so we can really just add this to summarization

Start in the summarization entry point that produces sources for declared.license and trace how license files are currently collected. Use the /licenses folder convention as the input case; done means multiple full license texts are included as sources and their declared licenses are combined with AND.

Written by the indexing model from the issue text.