Add support for virtual manifests

[R1kM]

When running Charon on a virtual manifest, e.g., serde, Charon currently crashes with the following error message

error: manifest path `/Users/fromherz/Work/papers/charon-tool-paper/tools/serde/Cargo.toml` is a virtual manifest, but this command requires running against an actual package in this workspace

[Nadrieril]

Can you try --cargo-arg '--package=serde' (with an appropriate package name)?

[Nadrieril]

I can confirm that works, so this is so far cargo working as expected. We'll have to investigate to see if we can ask it to translate all crates in the workspace.