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.
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.
