Add a test for trying to open a crate that has not yet been downloaded

[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.

As I write this up, I'm seeing that it would be easier if a function to read the Cargo.toml contents was extracted from the cargo_dir function and, instead, cargo_dir took the contents of Cargo.toml as an argument. That would make it easier to pass different Cargo.toml contents in for a unit test, so that should be part of this issue and should be done before adding the test.

I'd accept a PR that adds this test even if it fails, or if it fails with a weird error message! I'm not sure if I've coded things in such a way as to reuse existing cargo knowledge or not. Eventually I'd like this to panic with a message like "This crate has not been fetched yet; please run cargo install" first.

Bonus points if you change the code to make the test pass!

[marti1125]

count with me 👍