RFR 8217660: Refactor module related locked_create_entry_or_null() functions

[Harold Seigel]

Hi,

Please review this change for enhancement JDK-8217660.  The change does 
the following:

  * Replaces PackageEntryTable::locked_create_entry_or_null() with
    locked_create_entry_if_not_exist(), which has no return value.
  * Adds a PackageEntryTable::locked_create_entry() function for callers
    who know the package entry cannot already exist.
  * Replaces ModuleEntryTable::locked_create_entry_or_null() with
    locked_create_entry().
  * Adds a call to lookup_only(module_symbol) to check if a moduleEntry
    exists, instead of depending on locked_create_entry_or_null()
    returning null.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8217660/webrev/index.html

JBS Bug:  https://bugs.openjdk.java.net/browse/JDK-8217660

The fix was regression tested by running Mach5 tiers 1 and 2 tests and 
builds on Linux-x64, Windows, and Mac OS X, Mach5 tiers 3 -5 on 
Linux-x64, and by running JCK-12 Lang and VM tests on Linux-x64.

Thanks, Harold