Add prototype to generate `COPYRIGHT` from REUSE metadata
This PR adds a prototype to generate the `COPYRIGHT` file from the metadata gathered with REUSE. There are two new tools:
* `src/tools/collect-license-metadata` invokes REUSE, parses its output and stores a concise JSON representation of the metadata in `src/etc/license-metadata.json`.
* `src/tools/generate-copyright` parses the metadata generated above, (in the future will) gather crate dependencies metadata, and renders the `COPYRIGHT.md` file.
Note that since the contents of those files are currently incorrect, rather than outputting in the paths above, the files will be stored in `build/` and not committed. This will be changed once we're confident about the metadata.
Eventually, `src/etc/license-metadata.json` will be committed into the repository and verified to be up to date by CI (similar to our GitHub Actions configuration), to avoid having people install REUSE on their local machine in most cases.
You can see the (incorrect) generated files in https://gist.github.com/pietroalbini/3f3f22b6f9cc8533abf7494b6a50cf97.
r? `@pnkfelix`
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