packages: Optimize 'find-packages-by-name' to avoid disk accesses.

On a profile with 182 entries, "guix package --search-paths" goes from
4.5 seconds down to 0.4 second.

* gnu/packages.scm (find-packages-by-name): Make a name -> package vhash
  in a promise; access it with 'vhash-fold*'.