Enable depending on pkgs to re-use interfaces such as Backend

[absoludity]

Is your feature request related to a problem? Please describe.

The rust packages aren't available on crates.io. Particularly, I'd like to use the Backend trait but currently I either have to use the GH repo in my cargo.toml or copy the code in.

Just wondering is there a reason the packages aren't published on crates.io (or somewhere where they are published?)

Describe the solution you'd like

I'd like to be able to add my own implementation of Backend and test it out easily. I'm currently planning to test against the QIR SparseSim, but that also doesn't appear to be a published crate (I'm using the GH repo in my cargo.toml).

Describe alternatives you've considered

Using a tag with the GH repo in my Cargo.toml is what I'm doing currently.