reuse should stop smartly detecting the root of the project via VCS

[carmenbianca]

It's a nifty QOL feature, but it's high in astonishment, and I think it makes supporting fsfe/reuse-docs#90 slightly harder.

[mxmehl]

Hm, but that would cause errors when running reuse lint in a sub-directory of a project, right?

[mxmehl]

We agreed that the tool should continue being aware of the VCS root. Unlike other linting tools, REUSE's scope is strictly project-wide, and not limited to single files or sub-directories.

That is why the tool needs to pick up certain information that has to be in the root of the repository, most notably .reuse/dep5 and LICENSES, but also .gitignore. If the tool was unaware of this, users of a compliant repository would get a non-compliant result if they were running reuse lint in a subdirectory. We agreed that we don't want to confuse users this way.

Instead, we should fix bugs like #281 where the behaviour of lint and download differ, and provide the option to only lint specific directories and files.

[mxmehl]

Oh, forgot to note: @floriansnow brought up the idea of converting all paths internally to absolute first to lower the coding pain. @carmenbianca noted that this might make sense but certainly require a larger refactoring. If someone wants to work on this, please open a new issue.