Refactor input dumping and path retrieval with extension filtering

[mre]

Refactor the dump-inputs functionality to print all files checked for links and implement file path retrieval with extension filtering in the Input module.

I added this because it was hard to troubleshoot the extension changes, but I figured it would also be helpful to people who want to see which files would be checked when they write lychee ..

As an added benefit, dump-inputs now behaves more like the check command and properly ignores hidden files etc.

This is a follow-up to #1559.
Closes #1767.

[thomas-zahner]

Whew, that's quite a lot of changes. I didn't check every single detail nor did I test it locally. Are the existing tests covering the changes? I just see that you made a lot of changes and didn't add any tests but removed some.

[mre]

From my side, this is ready to be merged. I changed get_file_paths to get_input_sources, which yielded a much cleaner design. Thanks for the tip, @thomas-zahner.

The Docker image is currently failing because of #1774 and I hope it will be fixed with the next lychee release (0.20.0).

[thomas-zahner]

Looks great 👍 and actually fixes #1767

[mre]

Here I was, thinking that this would be a 10-minute change. So many edge-cases and caveats (and distractions, I guess)!
What a lift.

Anyway, this is finally ready now. As always, thanks for the thoughtful review @thomas-zahner.