Include licenses for Rust standard library

[jwodder]

Third-party crates aren't a Rust program's only dependencies; compiled Rust binaries also link with the std or core crate, both of which are dual-licensed under MIT and Apache 2.0, and thus their licenses need to be bundled as well. See rust-lang/rust#67014 for more information.

[sstadick]

Great point that I wouldn't have considered. That's not too hard to always add a core license I suppose, but we'd need to detect if something is no-std, which seems trickier? Leaving this as an open enhancement for now.

[BatmanAoD]

As a stop-gap, could the tool just manually copy over any top-level entries in the --previous file that it doesn't recognize (i.e. anything other than root_name and third_party_libraries)? This would allow users to manually add something like:

rust:
- package_name: std
  package_version: 1.82.0
  repository: https://github.com/rust-lang/rust
  license: MIT OR Apache-2.0
...