Get version for Wikibase cache key from the build

instead of basing it on core's version, as sometimes
we need to bump the key when updating extension
code, but that doesn't always correspond with core.

and when core changes and not wikibase, then the
same cache key should be okay to continue to use.

Change-Id: Ic4d4bd4170b42b49abd4650c113ed9419d8bd73e
Merge "Get version for Wikibase cache key from the build"