Fix online reindexing for plugins-dependent rules
Online reindex recalculates the status of all the changes
by invoking the Prolog rules associated.
That is good in general but breaks systematically when the
rules depend on plugins for their accurate evaluation.
Noteworthy example is the use of singleusergroup plugin in
conjunction with user-specific ACLs for labels.
Another even more severe example is the use of custom
Prolog predicates in rules.pl provided by the owners' plugin.
By swapping the order of registration of the Plugins module
and the Lucene Index module, the lifecycle listener will then
load first the plugins and only afterward activate the online
Bug: Issue 6472