Crates are incompatible with `cargo vendor`

Due to the `bevy_rapier2d` and `bevy_rapier3d` packages pointing to source files outside of the crate scope in `Cargo.toml`, attempting to use either of them with the `cargo vendor` command results in the following error:

`warning: fge_core v0.1.0 ignoring invalid dependency `bevy_rapier2d` which is missing a lib target`

One way to fix this would be to introduce a third crate (maybe `bevy_rapier_common`) which would then be added as a dependency for `bevy_rapier2d` and `bevy_rapier3d`.