RFR: 8241427: Coarsen locking in Modules::add_module_exports

[Claes Redestad]

Hi,

in some places we first lookup a PackageEntry - taking and releasing
Module_lock - only to then, after some trivial checks - take the
Module_lock again to update some state in the PackageEntry.

Coarsening the locking so we only take the mutex once per call brings a
small improvement to bootstrap (reduction of ~200-250k instructions)

Webrev: http://cr.openjdk.java.net/~redestad/8241427/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8241427

Testing: tier1-3

Thanks!

/Claes