rust: fix importCargoLock for repositories without toplevel Cargo.toml
rust: fix importCargoLock for repositories without toplevel Cargo.toml

Some crates do not have a Cargo.toml at the top-level, but only in
nested directories. Before this change importCargoLock used to fail with:

error: manifest path `/nix/store/some-store-path/Cargo.toml` does not exist

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs