Support multiple license files

[joshlf]

Cargo's license-file manifest key allows a crate author to specify the path to a license file. However, some projects have different license files applying to different files. Cargo should add a new license-files (plural) key which takes an array of file paths, which would be mutually exclusive with the license-file key.

Discussion on internals here.

[joshtriplett]

cc @infinity0 since this will also require an update to debcargo.

[infinity0]

Thanks, I'll keep an eye on it.

[ear7h]

I would like to take this on. Looks like it would be similar to 85d09d0 ?

[epage]

license-file is designed for custom licenses (non-SPDX). For anyone looking at this for including license files for SPDX files, see #12053.

For mapping files to licenses, something like REUSE is needed. We should consider how to make this work well with REUSE. For example, maybe license-file = "LICENSES/*" would be of interest but then that could include licenses that are SPDX.