Convert dev-dependencies to `path` dependencies

[bfops]

cargo publish gets mad at "cyclic" dependencies due to dev-dependencies.

The github issue (rust-lang/cargo#4242) claims that if you use path dependencies, they will be automatically ignored.

Definition of "Done"

• Update CI to complain about spacetimedb-* dev-dependencies that are not path dependencies
• Convert all our spacetimedb-* dev-dependencies to be path dependencies

[bfops]

I think we should do this quickly if it is, in fact, tiny.

[bfops]

Took maybe ~45 minutes all told