Keep dev-dependencies in sdist when there are no path dependencies

[messense]

Rewrite Cargo.toml without sync with Cargo.lock breaks building from sdist with --locked/--frozen

  cargo build --manifest-path Cargo.toml --message-format=json-render-diagnostics --target x86_64-unknown-linux-gnu --release -v --locked
      Updating crates.io index
       Running `git fetch --force --update-head-ok 'https://github.com/rust-lang/crates.io-index' '+HEAD:refs/remotes/origin/HEAD'`
  error: the lock file /home/conda/feedstock_root/build_artifacts/maturin_1675149830257/work/Cargo.lock needs to be updated but --locked was passed to prevent this
  If you want to try to generate the lock file without accessing the network, remove the --locked flag and use --offline instead.
  error: `cargo build --manifest-path Cargo.toml --message-format=json-render-diagnostics --target x86_64-unknown-linux-gnu --release -v --locked` failed with code 101
  error: subprocess-exited-with-error

See conda-forge/maturin-feedstock#84

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