Vendoring only mode to support build systems other than cargo

[ranfdev]

Naersk is good, but it imposes the use of cargo as a build system. Sometimes meson or make are needed.
If naersk had a vendor-only mode, it could be used alongside buildsystems other than cargo.

For example, to use meson to compile rust projects I need a

stdenv.mkDerivation rec {
  cargoDeps = rustPlatform.importCargoLock {
    lockFile = ../Cargo.lock;
  };
  buildInputs = [meson];
}

naersk could take the place of importCargoLock. The advantage is naersk can fetch git dependencies without me having to specify their hashes