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 ]

(From OE-Core rev: d3875a8111f1d88dcc83219d4700784050988ca3)

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

meta/classes/package.bbclass

@@ -123,6 +123,7 @@ def do_split_packages(d, root, file_regex, output_pattern, description, postinst
     dvar = d.getVar('PKGD', True)
     root = d.expand(root)
     output_pattern = d.expand(output_pattern)
+    extra_depends = d.expand(extra_depends)
 
     # If the root directory doesn't exist, don't error out later but silently do
     # no splitting.