Unlike regular input-addressed or fixed-output derivations, floating and
deferred derivations do not have their store path available at evaluation time,
so their outPath is a placeholder. The following changes are needed for
replaceDependencies to continue working:
* Detect the placeholder and retrieve the store path using another IFD hack
when collecting the rewrite plan.
* Try to obtain the derivation name needed for replaceDirectDependencies from
the derivation arguments if a placeholder is detected.
* Move the length mismatch detection to build time, since the placeholder has a
fixed length which is unrelated to the store path.
The tests cannot be directly built by Hydra, because replaceDependencies relies
on IFD. Instead, they are put inside a NixOS test where they are built on the
guest.
