Rust crate pack with --manifest-path

[msuozzo]

The current crates.io support uses --package to handle crates both with and without workspaces. Another option @orhun pointed out was to use the --manifest-path which appears to have better support and less variability across packages. Worth some further investigation.

[orhun]

I'm also happy to take a look if you can give me some pointers about how things work now vs. how you expect them to work :)