RFR: 8217450: Add PackageEntry::locked_lookup_only

[Claes Redestad]

Hi,

adding a PackageEntry::locked_lookup_only for use from places where
we know we hold the Module_lock avoids a small chunk of work during
startup.

Bug:    https://bugs.openjdk.java.net/browse/JDK-8217450
Webrev: http://cr.openjdk.java.net/~redestad/8217450/open.00/

Testing: tier1-3 (ongoing, complete on most platforms)

Thanks!

/Claes