For cleanliness the prelude module only needs to reexport preludes from
direct, "top-most" crates, which themselves take care of reexporting
preludes from its dependencies again. This shaves off some code while
maintaining the same set of exports.
This was temporarily allowed by Value trait refactoring, but the root
cause of the unused imports has been found (`glib::ToValue` for property
getters) and fixed in https://github.com/gtk-rs/gir/pull/1117.
gir has been fixed to omit all unnecessary imports from objects, enums
and flags which currently are the only offenders of this linter warning.
Hence disallow this warning from now on which is anyway not applied to
every crate - like gstreamer-gl - where such issues have been uncovered.
The next version of gir is going to generate doc(cfg()) attributes on
many symbols to show feature-dependence hints. While autogenerated sys
crates get this attribute in their own (generated) lib.rs file the safe
wrapper crates do not have such an autogenerated lib.rs file.
