This also changes stdenv's substitute --replace-fail to error out when
the replacement is the same as the search pattern, but can't be found.
This should not cause any problems in existing code, from what I can
tell from grepping nixpkgs.
The exception for pattern==replacement was previously introduced all the
way back in 5ff872aa24, but this was
apparently only used to make the check for the warning "simpler".
replaceVars has a checkPhase to confirm that no left-over @...@ patterns
remain. Since it's not possible to use "placeholder" with replaceVars,
the substitution of some patterns must be delayed to a later step.
By passing "null" for those keys explicitly, replaceVars can make an
exemption *just for this case*, but keep checking all other references.
