multilib/recipes: Use new RecipePostKeyExpansion event

There are issues with multilib due to the ordering of events where some
functions see the remapped multilib dependencies and some do not. A significant
problem is that the multilib class needs to make some changes before key expansion
and some afterwards but by using existing event handlers, some code sees things
in a partially translated state, leading to bugs.

This patch changes things to use a new event handler from bitbake which makes the
ordering of the changes explcit.

The challenge in doing this is that it breaks some existing anonymous python and
dyanmic assignments. In some cases these used to be translated and no longer are,
meaning MLPREFIX has to be added. In some cases these are now translated and the
MLPREFIX can be removed.

This change does now make it very clear when MLPREFIX is required and when it is
not, its just the migration path which is harder. The patch changes the small number
of cases where fixes are needed.

In particular, where a variable like RDEPENDS is conditionally extended (e.g.
with an override), MLPREFIX is now required.

This patch also reverts:
base: Revert 'base.bbclass: considering multilib when setting LICENSE_EXCLUSION'

This reverts 65971302 as the changes
to multilib datastore handling mean its no longer necessary.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>