Add a test for trying ot open a crate that is not a dependency of the current project

[carols10cents]

In main.rs, add a unit test for the cargo_dir function that checks that it panics if we have specified a new crate in the Cargo.toml file but have not yet run any command that would fetch that crate's source to our local filesystem.

Use the existing test marked should_panic for an example of how to make a test check that a function panics.

I doubt that this test will pass currently! Eventually, I'd like this to panic with a message like "Cannot open crate whatever because it is not a dependency of the current project`" or similar.