Realising a missing output of a derivation with another output already valid

[shlevy]

If derivation foo has outputs bar and baz and foo.baz is valid in the store but foo.bar isn't, there is currently no way to build foo.bar without invalidating foo.baz first. When chroot builds are enabled, we can just rebuild everything in the chroot, but when they aren't possible some form of hash rewriting will be necessary.