Disallow import of non-deprecated name

The `GlSkeletonLoading` component was deprecated in favour of
`GlSkeletonLoader` as part of
https://gitlab.com/groups/gitlab-org/-/epics/4301.

This makes sure that only the `GlDeprecateSkeletonLoading` import name
can be used, to remove any ambiguity about whether or not it's
deprecated.

Part of https://gitlab.com/gitlab-org/gitlab/-/issues/357001.