rustdoc: Populate external_traits with traits only seen in impls
This means default methods can always be found and "Important traits" will include all spotlight traits.
* Visibility was missing from impl items.
* Attributes and docs were missing from consts and types in impls.
* Const default values were missing from traits.
This unifies the code that handles associated items from impls and traits.
Currently if a `#[doc(hidden)] pub use` item is inlined the `hidden`
attribute is ignored so the item can appear in the docs. By never inlining
such imports, they can be stripped.
Instead of finding aux-build files in `auxiliary`, we now search for an
`aux` directory relative to the test. So if your test is
`compile-fail/foo.rs`, we would look in `compile-fail/aux`. Similarly,
we ignore the `aux` directory when searching for tets.
This used to be done to avoid inlining impls referencing private items,
but is now unnecessary since we actually check that impls do not
reference non-doc-reachable items.
This changes the current rule that impls within `doc(hidden)` modules
aren't inlined, to only inlining impls where the implemented
trait and type are reachable in documentation.
