Bug 15743: Allow plugins to embed Perl modules

Bug 15743: Allow plugins to embed Perl modules

This patch allows plugins to embed Perl modules by ignoring Perl modules
in plugins directory that don't inherit from Koha::Plugins::Base

TEST PLAN
---------
1) Edit koha-conf.xml to have enable_plugins set to 1, and
   the pluginsdir set to a particular path.
2) Create a dummy plugin file. (e.g. {that path}/Koha/Plugin/Kaboom.pm)

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com
(cherry picked from commit 2a43195322c228795b08ebbc165e360c725c2b56)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>