reuse lint should check existence of folders/files listed in dep5 file

[misappi]

Currently, reuse lint does not complain if in the dep5 file folders and/or files are listed that do not exist. If for example, this would be an excerpt on a dep5 file

...
Files: bubbelebu/*
Copyright: ...
License: 
...

but the folder bubbelebu would not exist, reuse lint wouldn't complain about that. But especially, if you have a lot of third-party stuff in your project and a complex dep5 file, it would make sense that reuse lint did these kind of existence checks to keep the dep5 file consistent to the code.

[mxmehl]

I agree, thanks for noticing that!