Only call mro_package_moved on new substashes

Only call mro_package_moved on new substashes

Commit 298d65111 added this mro_package_moved call.

It does not need to happen if the substash already exists, as it will
already have had effective names assigned to it.

It also may not be a good idea to set it in such cases, as it may make
a recursive call to mro_get_linear_isa. I know this is utter paranoia,
but someone may write a mro plugin that is not reëntrant. (The speed
gain is worth it, though.)