make missing_doc lint respect the visibility rules

Previously, the `exported_items` set created by the privacy pass was
incomplete. Specifically, it did not include items that had been defined
at a private path but then `pub use`d at a public path. This commit
finds all crate exports during the privacy pass. Consequently, some code
in the reachable pass and in rustdoc is no longer necessary. This commit
then removes the separate `MissingDocLintVisitor` lint pass, opting to
check missing_doc lint in the same pass as the other lint checkers using
the visibility result computed by the privacy pass.

Fixes #9777.
auto merge of #10277 : dcrewi/rust/missing-doc-and-visibility-rules, r=alexcrichton

Now the privacy pass returns enough information that other passes do not need to duplicate the visibility rules, and the missing_doc implementation is more consistent with other lint checks.