[400221] More extensible mergers... and use extensions for requirements

[400221] More extensible mergers... and use extensions for requirements

We've made the mergers extensible through bug 398863... but we do not
make use of these extensions when merging other diffs from within the
merging process (merging requires/requiredBy, merging equivalences...).

This patch transforms the merger mechanism to make sure we always use
the extensions instead of Diff.copyRightToLeft or Diff.copyLeftToRight;
and depreciates these two methods. All of the merging code has been
moved to three new specific mergers that will be easier to extend. These
three 'default' mergers are provided through the 'mergerExtension'
extension point which has been moved to the o.e.c.ide plugin.

This also changes all tests so that they use the new APIs instead of the
Diff.copy* methods.

Bug: 400221
Change-Id: I41bb9ab74001e0c94ecb26ffec88cecf599a67be