[12] RFR JDK-8167314: Enable the check to detect duplicate provides in in GenModuleInfoSource

[mandy chung]

A simple patch to enable the check to enforce no duplicate provides
in module-info.java.extra, same checks as javac.  This check
was disabled in the fix for JDK-8202941 because of duplicated
provides generated for jdk.internal.vm.compiler.  Since JDK-8208463
is now resolved [1], we can enable this check.

Webrev:
   http://cr.openjdk.java.net/~mchung/jdk12/webrevs/8167314/webrev.00/

Mandy
[1] http://hg.openjdk.java.net/jdk/jdk/rev/c5461fe16efb