Fix leaking non-public 'use' statements with glob imports

Working on #4366
Fix leaking immediate children and types via glob imports

More work on #4366
Fix implicit leaks of imports throughout libraries

Also touch up use of 'pub' and move some tests around so the tested functions
don't have to be 'pub'

Closes #4366
auto merge of #5113 : alexcrichton/rust/issue-4366, r=catamorphism

The first two commits are the actual fix going into place, and the last is just dealing with the fallout in the rest of the compiler.

The test added in the first two commits had 0 failures before this patch, and if the glob imports were changed to explicit imports then the errors showed up. Due to this behavior, I figured that the desired behavior was for glob imports to not accidentally leak a lot of non-public imports/functions/types into other modules.

There was quite a lot of fallout, and it all passes `make check` locally, but I doubt that it will pass on all platforms because there's probably some guarded off thing I missed.

I plan on making another patch soon which changes the default level of `unused_imports` to `warn` which will hopefully reduce a lot of the `use` noise throughout. In conjunction with #5104, and a few minor fixes, I think that the default level of `warn` is actually really usable.