classes/lib: Switch classextend to use new filter API

classes/lib: Switch classextend to use new filter API

Currently, class extensions are implmented using shadow variables and
access indirection which is horribly ineffient and ugly.

Switch to using the new bitbake filter API, which allows a translation
of the variable before the expanded value is returned. This allows us
to drop the shadow variable accesses. It also avoids the need to iterate
PACKAGES and make many variable changes since a filter against RDEPENDS
applies to RDEPENDS:${PN} and all of it's other overridden values.

Since data expansion happens at access, it also avoids many of the race
conditions this code has tranditionally been plagued with.

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