Add support for Monorepos

[ypid]

Imaging a monorepo like this:

monorepo-example
├── project_a
│   ├── README.md
│   └── .reuse
│       └── dep5
├── project_b
│   ├── README.md
│   └── .reuse
│       └── dep5
└── README.md

Without this change, reuse is confused that at the root of the project, there is no .reuse directory structure.

This PR probably needs discussion before it can be merged. I just went ahead and provided this POC implementation so that we have some code to talk about. At the very least, a test will need to be added. Refer to https://github.com/ypid/latex-packages for a real life (mono)repo for testing.

[mxmehl]

Phew, thanks for bringing that up. However, I am a bit afraid of adding this as it would complicate the spec and also the tool. In our spec, we only allow for one .reuse directory: https://reuse.software/spec/#dep5

However, we're working on supporting more flexible files for bulk-licensing stuff that shall soft-deprecate the DEP5 files. See more details here: spdx/spdx-spec#502

If we introduced that, it should solve your issue, and we can dep5 deprecate slowly. What do you think?

[ypid]

The proposal from https://lists.fsfe.org/pipermail/reuse/2020q3/000085.html sounds good. It would address it. You can close this if you want.

[mxmehl]

Thanks for the info. I hope we'll have this implemented soon :)

[adisbladis]

I struggled with adopting reuse for a monorepo I am hacking on with quite a complex set of licenses and ended up writing a tool that invokes reuse addheader ... and generates a dep5 file.

Quoting myself from my commit message:

Writing reuse dependency files is not a fun excercise in a complex repository with multiple
sub packages with a complex set of different licenses and sometimes even different licenses
within the same directory.

For example we license Nix expressions under MIT no matter where in the tree they live
so that they can freely be copy-pasted into nixpkgs and other third party repos
while the most important bits are all GPL-3.0-only.

This script provides a way to express our rules and outputs a reuse compatible dep5 file

It's quite a different approach to licensing management, instead of checking that files have licensing attached it declaratively manages the entire repository licensing.

@mxmehl Would you be interested in integrating something like this into reuse itself?
I would be happy to re-license my code if it's interesting to the reuse project.

Btw, just to clarify: Of course I'm not suggesting that users should write rules in Python, it was just the lowest mental overhead approach while I was hacking on it.
A more suitable format for a more generally useful tool would be something like TOML.

[ypid]

I just noticed that the use case I initially opened this PR for does not need code changes. Using --root for example reuse --root . lint in project_a works nicely.