Perform most diagnostic lookups in `resolution_failure`

Previously, these were spread throughout the codebase. This had two
drawbacks:

1. It caused the fast path to be slower: even if a link resolved,
rustdoc would still perform various lookups for the error diagnostic.
2. It was inconsistent and didn't always give all diagnostics (https://github.com/rust-lang/rust/issues/76925)

Now, diagnostics only perform expensive lookups in the error case.
Additionally, the error handling is much more consistent, both in
wording and behavior.

- Remove `CannotHaveAssociatedItems`, `NotInScope`, `NoAssocItem`, and `NotAVariant`
  in favor of the more general `NotResolved`

  `resolution_failure` will now look up which of the four above
  categories is relevant, instead of requiring the rest of the code to
  be consistent and accurate in which it picked.

- Remove unnecessary lookups throughout the intra-doc link pass. These
are now done by `resolution_failure`.
  + Remove unnecessary `extra_fragment` argument to `variant_field()`;
    it was only used to do lookups on failure.
  + Remove various lookups related to associated items
  + Remove distinction between 'not in scope' and 'no associated item'

- Don't perform unnecessary copies
- Remove unused variables and code
- Update tests
- Note why looking at other namespaces is still necessary
- 'has no inner item' -> 'contains no item'

bless tests

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