Fix memory leaks in mro_package_moved

Fix memory leaks in mro_package_moved

This commit adds a new HV_FETCH_EMPTY_HE flag for hv_common. It is to
be used in conjunction with HV_FETCH_LVALUE. It just stops the newly-
created HE from having a new undef scalar assigned to it.

This allows code to call hv_common just once instead of an hv_exists/
hv_store pair.

It was such a double hv_common call that I was trying to avoid with
HV_FETCH_LVALUE, without realising that it was leaking.