populate: stop if a flake's URL can't be found

The URLs of flakes went into nix flake lock's and nix flake metadata's
arguments as command substitutions, whose failure doesn't stop a script
under set -e.  After "error: source p is not a directory", nix flake
lock ran with an empty URL, which Nix takes for the current directory:
on a remote target, the login directory, which ended up in the
world-readable store, .ssh included.  URLs get assigned first now, so
failing to find one stops populate.

A phase's unexpected lists what populate is expected not to print,
whether it succeeds or not; notdir expects populate not to go on
locking.

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