driver: Modules importing GHC.Prim are always recompiled

HEAD only

[nix-shell:~/ghc-hash-builds]$ cat Pr.hs 
module Pr where

import GHC.Prim

[nix-shell:~/ghc-hash-builds]$ ../ghc-clean/_build/stage1/bin/ghc Pr.hs 
[1 of 1] Compiling Pr               ( Pr.hs, Pr.o )

[nix-shell:~/ghc-hash-builds]$ ../ghc-clean/_build/stage1/bin/ghc Pr.hs 
[1 of 1] Compiling Pr               ( Pr.hs, Pr.o ) [ghc-prim removed]

This is due to my driver refactoring patch. I have a fix.

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