populate: add flake source type

Deliver a flake, or one of its locked inputs, to the target: archive the
flake locally, copy the selected input together with its own inputs into the
target's store, and link it at the target path, registered as garbage
collector root.  The result can be used in NIX_PATH as well as by flakes
evaluated on the target, without network access.

nix copy runs with a separate ssh wrapper without the control socket, because
Nix waits for its LocalCommand, which may not run over a multiplexed
connection.

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