Non exported paths can be imported from other crates if they are reachable

[msullivan]

The metadata and resolve3 don't distinguish between things in the metadata because they were actually exported and things that are in the metadata because they were reachable. This means that a lot of things can be imported from other crates that weren't actually exported.

The solution is probably to tag whether something is exported in the metadata and check that in resolve3.

[nikomatsakis]

Not critical for 0.6; removing milestone

[catamorphism]

This is fixed.