rustdoc: report unresolved paths that appear invalid

- Check whether an unresolved path is in fact invalid. This prevents rustdoc
  from emitting nonsensical diagnostics like:

  - "no item named `std:` in scope"
  - "no item named `` in scope"

  Instead rustdoc will now say "has invalid path separator" (the same message
  from `MalformedGenerics::InvalidPathSeparator`)

  This is done by checking whether each path segment is empty or contains extra ":"
  after it's been split by the path separator "::".

- Add a dedicated test `tests/rustdoc-ui/intra-doc/invalid-path-separator.rs`
  for this.

- Some unrelated test snapshots have been updated because of this new check.