feat: Remove the public API entry

[alexis-opolka]

We are removing the public API entry (the lib.rs file) which made it possible to have navi as a library dependency.

After some thoughts and consideration, no one is currently using navi as a dependency (see crates.io/reverse_dependencies) and the exposed entries are barely the minimum that would be required to work with navi.

This also causes issues with downstream packaging on platforms such as Fedora where they create sub-packages because we have a public API.

As of now, navi is not suited to be used as a rust dependency and doesn't seek to become a stable and reliable rust library. If the need ever arises in the future, we might expose once again an API, though.