DFG v1 can only `@reexport` upstream v1+ features

[dehann]

Contrast, if an upstream package is not stable and break their exports, then DFG v1 would automatically also break, which is not really acceptable. So we need to add the restriction that only upstream stable v1's can be reexported.

This came as @Affie and I spoke -- pretty important.

[Affie]

Re-exports were removed