Fix GotoDefinitionHelpers.TryGoToDefinition helper method to not assume that the document for the resultant definition is within the same workspace as the original document. An external result provider might return a navigable item with location in a document in another workspace.

Initial work to add support for external language service result providers for Find All References, GotoDefinition and NavigateTo.

All the newly added result provider interfaces are internal, and would eventually go through API design review to enable third party plugin implementations.

* Remove unintentionally added extra option.
* Rename NavInfo property to NavInfoFactory.
* Add fallback to metadata-as-source if NavInfo can't be retrieved for some reason.
* Use [ExportLanguageService] attributes rather than language service factories.
* Add NavInfo tests for generic methods and types
* Add negative NavInfo tests for parameters, locals and labels
* Rename UsePreviewTab option to PreferProvisionalTab to remove some confusion around what it does, and document it.

This requires some updated handling of our INavigableItem interface.
The Visual Studio implementation previously would only show the
DisplayString if it had children, and would otherwise show file
location and the source snippet. For implementations, the source
snippet is somewhat useless: in the case of methods you just see
the method signature a bunch of times. Thus, we wanted to show source
location and a fully qualified name, something which wasn't expressible.

The new interface now has DisplayString, which can be null if you want
the string to be line from the source. Whether you want to show the file
location is explicitly called out now too.

TryGoToDefinition accepted a containingTypeSymbol which was used
to do some specific logic to see if third-party navigation is allowed.
Move that logic to the AbstractGoToDefinitionService and just pass the
result of the logic through.

Pass CancellationTokens through from Go To Definition. Pass cancellation tokens through type and namespace generation,
since those operations can do large amounts of work.

Pass CancellationTokens through from Go To Definition. Pass cancellation tokens through type and namespace generation,
since those operations can do large amounts of work.