Preserve the parent module of `DocFragment`s

- Add `parent_module` to `DocFragment`
- Require the `parent_module` of the item being inlined
- Preserve the hir_id for ExternCrates so rustdoc can find the parent module later
- Take an optional `parent_module` for `build_impl` and `merge_attrs`.
  Preserve the difference between parent modules for each doc-comment.
- Support arbitrarily many re-exports in from_ast. In retrospect this is
  probably not used and could be simplified to a single
  `Option<(Attrs, DefId)>`.
- Don't require the parent_module for all `impl`s, just inlined items

  In particular, this will be `None` whenever the attribute is not on a
  re-export.

- Only store the parent_module, not the HirId

  When re-exporting a re-export, the HirId is not available. Fortunately,
  `collect_intra_doc_links` doesn't actually need all the info from a
  HirId, just the parent module.

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