Support for generating third-party licenses file from `deny.toml`

[joshtriplett]

The list of licenses in deny.toml is also the list of third-party licenses I'd want to reproduce in a THIRD-PARTY-LICENSES.txt file. I'd love to have a feature in cargo deny to generate such a file based on deny.toml.

(Note that that shouldn't be a complete list of licenses, since deny.toml lists enough licenses to satisfy all the SPDX clauses, and if you have a crate licensed as MIT OR AwfulLicense, you don't want to list AwfulLicense.)

[Jake-Shadle]

This exists in cargo-about and there has already been interest in combining them EmbarkStudios/cargo-about#201 which would be some amount of work, but would ultimately result in one less project I maintain so it's definitely something I want to do at some point when I have the time.

[Jake-Shadle]

FWIW cargo-about already does what you want in terms of only using the minimal set of licenses needed to satisfy the expression, but it does that by requiring that the allowed licenses in your config are ordered by priority, so eg. if a crate has Apache-2.0 OR MIT and the config is ["MIT", "Apache-2.0"] then MIT would be the license used by that particular crate.