Breaks the internal GTD infrastructure into seperate symbols for
determining the navigable symbol at the caret position and performing
the navigation.

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.

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.