Non exported enum variants can be imported from other crates

[msullivan]

Non exported enum variants can be imported from other crates, although not by modules in the same crate.

This is because enum variants are unconditionally written out to metadata.

[msullivan]

This is somewhat related to #3137.

[msullivan]

Static trait methods are going to have the same problem for the time being.

[catamorphism]

#4082 supersedes.