derivations: Use a regular hash table for the module cache.

The hit rate of the 'add-data-to-store' cache goes from 10% to 4% on:

  guix build -e '(@@ (gnu packages libreoffice) libreoffice)' -nd

* guix/derivations.scm (%module-cache): Turn into a regular hash table.
It didn't make sense to use a weak-value hash table given that values
are pairs.

guix/derivations.scm

@@ -1208,7 +1208,7 @@ they can refer to each other."
 
 (define %module-cache
   ;; Map a list of modules to its 'imported+compiled-modules' result.
-  (make-weak-value-hash-table))
+  (make-hash-table))
 
 (define* (imported+compiled-modules store modules #:key
                                     (system (%current-system))