populate: generate flakes for inputs drawn from sources

For each source that draws inputs from other sources, populate generates a
flake in .populate/flakes/<name>, which re-exports the outputs of the
source's flake with those inputs drawn from the other sources.  The source
itself stays as it is, so NIX_PATH still refers to it.

Generated flakes refer to each other's generated flakes, so a source's own
declarations apply wherever it gets drawn from, and get generated in
dependency order.  Evaluation fails for sources drawing inputs from missing
sources, from themselves, or from each other.  Generated flakes get locked on
every run, and get removed once their sources no longer draw inputs from
other sources.

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