Display ignored submodules in summary

[mxmehl]

Picking up an element of fsfe/reuse-docs#36.

The tool should/could communicate the number of ignored submodules and their respective folder names/paths in the Summary section. Otherwise, it could be harder for a user to understand why a project on their computer is compliant although it contains uncompliant files.

Currently, this is only displayed in the debug logs (e.g. reuse.project - INFO - ignoring 'inventory' because it is a submodule).