`cargo-gpu` can't find `Cargo.toml` from a package specified with `cargo build -p {package}`

This is because internally I treated `-p` as a synonym for `--shader-crate`, but the latter is a _path_ while the former specifies the **name of a package**. 

These actually have to be distinct parameters with separate handling. `-p` needs to do a lookup in the workspace for the package to resolve the path to the crate.