If meal is specified as an unauthenticated https github or gitlab repo, download it as tarball automatically.

`builtins.fetchTree` supports `rev` as a reproducibility indicator (no need for extra hash), so that works fine for us.

Also, this way we can avoid certain cases where https://github.com/NixOS/nix/issues/5119 slows things down too much (not al cases though 😕).

BREAKING CHANGE: `mrchefCombinedSource` now gets an `optimizeMealFetch` argument instead of `shallow`.