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.