derivations: 'build-expression->derivation' caches its module derivations.

This reduces the number of lookups in the 'add-data-to-store' cache from
7505 to 3329 (hit rate from 68% to 27%) when running:

  GUIX_PROFILING=add-data-to-store-cache guix build libreoffice -nd

The execution time of "guix build libreoffice -nd" goes from 2.12s to 1.87s.

* guix/derivations.scm (%module-cache): New variable.
(imported+compiled-modules)[key]: New variable.
Lookup KEY in %MODULE-CACHE and populate %MODULE-CACHE upon cache miss.