package.bbclass: check packages name conflict in do_package

package.bbclass: check packages name conflict in do_package

It is possible to rename packages with the macro PKG:${PN} and result in
a package name conflict if the target name exists already.

Add a fatal check to prevent this issue to go unnoticed.

Fix [Yocto #12060]

Reviewed-by: Yoann CONGAL <yoann.congal@smile.fr>
Signed-off-by: Fawzi KHABER <fawzi.khaber@smile.fr>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>