tests: add edge cases of flakes

A new test, edge-cases, covers paths that work but have no scenario yet:
a flake given by an indirect reference, which the target fetches by its
locked reference; a remote flake in a subdirectory; a flake in a
subdirectory drawing inputs from sources; a dirty Git work tree; a flake
whose lock file needs updating; drawing inputs from a source that isn't
a directory; nested names of missing inputs; several sources sharing one
flake; and an input locked to a path that the target has.

The deploying machine gets a registry entry for the indirect reference.
Resolving it fails if the global registry can't be downloaded, which it
can't in the VM, and disabling the global registry with flake-registry
needs flakes, so the deploying machine enables them, like any deploying
machine using indirect references does.  The target doesn't, so checks
for flake support on targets still mean something.

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