PreloadingItemGenerator: process normal pages

The item preloading generator only prefetches if the input
generator yields WikibasePage, with a Site class that
has method preloaditempages.

This allows a Page which is in the 'wikibase-item' namespace to be
be preloaded.

Bug: 54999
Change-Id: I62140a91bb68a67a93dcf7bb70bfe284de672d02
Merge "PreloadingItemGenerator: process normal pages"