packages: Disambiguate 'modules' and 'imported-modules' in <origin>.

packages: Disambiguate 'modules' and 'imported-modules' in <origin>.

The two mistakes made here (confusion between 'modules' and
'imported-modules') were canceling each other.

* guix/packages.scm (patch-and-repack): Use IMPORTED-MODULES, not
MODULES, as the base of the module list passed as #:modules to
'gexp->derivation'.
(origin->derivation): Pass IMPORTED-MODULES, not MODULES, as
the #:imported-modules argument of 'patch-and-repack'.
* gnu/packages/engineering.scm (fastcap)[source]: Add 'imported-modules'
field.