gexp: Cache the module to derivation mappings.

This reduces the number of 'add-data-to-store' cache lookups from 3329
to 2743 (hit rate: 27% to 11%) when running:

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

Execution time of "guix build libreoffice -nd" goes from 1.86s to 1.80s.

* guix/gexp.scm (imported+compiled-modules): Wrap body in 'mcached'.