Add support for packaging crates from git

It would be useful if it was possible to also package crates that are not on crates.io but on a provided Git location, similar to what cargo install allows.

My main motivation is the packaging of software that is using the Rust ecosystem but is not open or just not on crates.io, in particular this involves a company setting (and the currently virtually absent support for having your own crate repositories).

I know that the packaging of closed software is and should not be a priority for Debian, but given that it has always been possible to build on Debian/Ubuntu by using all its tools, it might be something nice to have.

I don't know enough about debcargo's internals to know if it is just a matter of adding command parameters and passing them to cargo or if it is much more complicated than that, but it is almost certain I need to work on this. This issue is meant as much as a discussion topic as a feature request.