add option to exclude hidden dirs and files

[gaelgatelement]

Reasoning : In our CI, the build artifact contains dependencies in a hidden directory. Because of this, reuse fails to validate using lint because its detecting issues in the dependencies directory.

By adding this option, we are able to explicitly tell reuse to exclude hidden directories, which contains dependencies in our case.

[mxmehl]

Thank you for your submission. It would have been great if you had created an issue beforehand, as we will most likely not accept this PR. Let me explain why:

We don't want to add flags to lint that make a difference on the compliance status, especially if they make it easier to reach compliance

As per specification, a repository can be either REUSE compliant or non-compliant. All exceptions, ignored files etc. are equal for everyone, so it doesn't make a difference whether you, I, a CI or the API lint the project. Only the output can be different (like JSON or no output).

The problem with flags that can alter the compliance status is that there would be multiple compliance definitions, based on the custom flags or hacks that one applies locally. That's not wanted and would render the actual goal of REUSE void: make licensing and copyright easy and understandable for humans and machines alike.

Side note 1: we should document this somewhere as this question appears quite often

Side note 2: --include-submodules and --include-meson-subprojects may have an effect on compliance, but these non-defaults make reaching compliance harder, not easier.