feat: Add support for --dump-inputs

[Techassi]

Fixes #684

This PR adds support for the CLI flag --dump-inputs which prints out input sources without collecting and checking links.

[mre]

Thanks for tackling this! Very nice and clean implementation.
The only thing I'd add would be a unit and/or integration test, to make sure it produces the expected output. Let me know if you need some support with that, but I guess you got this.

[Techassi]

I added some integration tests in ac3fa41. I think this PR is now good to go!

[mre]

Great! I made a minor change to run the integration tests from a newly-created fixtures/dump_input subfolder. The reason being, that it's easier to keep the test fixtures isolated this way. Otherwise, we'd have to update the dump_input tests if we moved files around for other tests.

Hope that's fine. Looks good to me now. 😃

[Techassi]

Yes, that's fine! Moving these test files into their folder for the obvious reasons you mentioned makes sense 😄 Thank you for merging this super quickly!

EDIT: I created an upstream PR to test if a string contains all needles. This is especially useful when checking multiple lines of output. If this gets merged, we can simplify the tests quite a lot.