Helper to analyze Debian reverse dependencies

This helper tool fetches and parses Debian archive metadata to figureout which packaged crate versions are used by which packages.

It looks for packages matching the Debian Rust Team policy naming scheme

• source packages named rust-<crate>[-<ver>]
• binary packages named librust-<crate>[+<feature>][-<ver>]-dev

and tries to detect the following package relationships

• build dependencies from any source package to binary crate packages
• regular dependencies from any binary package to binary crate packages
• debci/autopkgtest triggers from any source package to binary crate packages
• Static-Built-Using from any binary package to source crate packages

including "orphaned" relationships which are not satisfiable by package versions currently in the archive.

When debugging, setting RUST_LOG=debug produces a lot more output.

This helper is still in early stages. It does already handle most Rust-packaging specific scenarios better then list-rdeps.sh in debcargo-conf, dak and other stock Debian tools.

Bug reports and improvements more than welcome!