populate: deliver flakes and draw inputs with the Rust program

The program delivers flake and flake-input sources, draws inputs from
sources, and removes state that sources don't need anymore, so it does
everything the shell implementation does, and passes all scenarios.
src/flake.rs builds the same shell code as shell.nix generates for
these, with every value quoted, so both implementations behave alike
while that code moves to Rust piece by piece.

The ssh scripts in the temporary directory run the ssh that was in PATH
when populate started, so one can be ssh in PATH for nix copy.  The
other one is for nix copy's ssh:// store, without the control socket.

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