Before this change `SymbolInformation` provided by a document was the
info for all encountered symbols that have not yet been emitted. So,
the symbol information on a `Document` was a mishmash of symbols
defined in the documents, symbols from other documents, and external
symbols.
After this change, the `SymbolInformation` on documents is just the
locals and defined symbols from the document. All symbols referenced
and not from emitted documents are included in `external_symbols`.
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