radicle/git/raw: Limit scope of `RemoteCallbacks` — heartwood

Found with cargo clippy --package=radicle, this is only used in tests.

 // Re-exports that are only used within this crate.
 pub(crate) use git2::{
     message_trailers_strs, AutotagOption, Blob, Config, FetchOptions, FetchPrune, Object,
     RemoteCallbacks, Revwalk, Sort,
     message_trailers_strs, AutotagOption, Blob, Config, FetchOptions, FetchPrune, Object, Revwalk,
     Sort,
 };
 // Re-exports that are only used within this crate for testing.
 #[cfg(any(test, feature = "test"))]
 pub(crate) use git2::RemoteCallbacks;
 // Re-exports that are used by other crates in the workspace, including this crate.
 pub use git2::{
     Branch, BranchType, Commit, Direction, Error, ErrorClass, ErrorCode, FileMode, ObjectType, Oid,