RFR: 8217450: Add PackageEntry::locked_lookup_only

[Claes Redestad]

On 2019-01-22 23:00, Lois Foltan wrote:
> One more minor note though, you could remove the check in the assert for 
> pkg != NULL, since the locked_lookup_only will always indicate if a 
> PackageEntry was found whether it was already there or just newly created.
> Lois

Right, we went back and forth on this elsewhere in the review thread and
ended up putting the pkg != NULL check back in to not unnecessarily
penalize fastdebug build performance.

/Claes