fix(pkg): unknown packages should have variables evaluate to empty

[Alizter]

As described in

• test: variables of undefined packages #13465

We fix the expansion of variables of unknown packages to become the empty string.

Part of the work on

• Relocatable compiler in dune #13229

[Leonidas-from-XIV]

I don't know if it is the right way to go about it, there was some prior work on capturing undefined variables and defining them. I think it was mainly to contain the undefined variable semantics from OPAM to lockfile generation, whereas undefined variables on the Dune side would always error.

[rgrinberg]

If we know the package isn't in the computed lock directory, we should actually evaluate this variable when the lock directory is solved, not at build time.