Emit definitions and references for included files

[varungandhi-src]

We should emit:

• A reference for #include directives.

• A definition for the included file.

  Normally these are just headers, but in principle, one can include any files at all.

Together, these would make Go to Definition from the #include to the included file work.

• We should avoid emitting redundant definitions for the same file.
• It's not super clear what the source location for the definition should be. I can't recall if there is a way to encode a reference to a file instead of a specific source range in the file.
  • If the former is possible, we should do that.
  • If a specific source range is needed, that makes it trickier; I can't think of a way to pick a non-empty range off the top of my head that works reliably and gives intuitive results. Picking an empty range is an option.

Note that you can't get a Code Intel popover with Find References for a file if we don't pick a source range (or use an empty range); you'd have to do a text search like #include .*MyFile\.myext, which is a little unfortunate. As an approximate solution, perhaps we should use the first line if it is blank or a comment?

Supporting Go to Definition from #include directives was flagged as a customer issue. They didn't flag Find References specifically.

For context, we do support this functionality for imports in Go today.

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Go has explicit package declarations, which nicely resolves the issue of Find References.