[INTERNAL DESTINATION] fixed duplication of packages (#1908)
[INTERNAL DESTINATION] fixed duplication of packages (#1908)

while a package was going through internal destination, it was sent to
`duplicate_content` which is also duplicating items of the packages, but
those items were already published individually before.

This patch fixed it by calling directly `duplicate_item` which is not
duplicating items of the package.

fixes SDESK-5371
petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core,petrjasek/superdesk-core,petrjasek/superdesk-core,superdesk/superdesk-core,superdesk/superdesk-core