package: do_split_packages: expand variables in extra_depends

package: do_split_packages: expand variables in extra_depends

If a recipe passes for example extra_depends="${PN}-dev" to do_split_packages()
then it isn't expanded, so the check for the multilib prefix doesn't work.
Solve this centrally by expanding extra_depends inside do_split_packages().

[ YOCTO #9381 ]

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>