Cross-repo Find implementations does not work when interface is defined downstream of type

[varungandhi-src]

There was a code example shared here:

type Foo interface {
	Bar()
}

https://demo.sourcegraph.com/github.com/simoncwaterer/example1code/-/blob/example1.go?L5:6-5:9#tab=implementations_go

The example1code repo depends on example2code which has a type which conforms to this interface:
https://demo.sourcegraph.com/github.com/simoncwaterer/example2code/-/blob/ex/ex.go

type T struct{}

func (t *T) Bar() {
	fmt.Println("Bar")
}

So the interface is defined "downstream" of the type. In this case, Find implementations on the interface does not give T as one of the results.

[varungandhi-src]

There is a TODO for this in the code:

		// local type -> local interface
		findImplementations(localTypes, localInterfaces, symbols)

		// local type -> remote interface
		findImplementations(localTypes, remoteInterfaces, symbols)

		// TOOD: We should consider what this would even look like?
		//     I don't think this makes sense the current way that we are emitting
		//     implementations. You wouldn't even catch these anyways when uploading
		// remote type -> local interface
		// findImplementations(remoteTypes, localInterfaces, symbols)

I think there are a couple of different ways to make this work.

1. We emit an external symbol for the type and attach the relationships there. Make sure that relationships on external symbols are handled correctly in the backend.
2. Add a new IsImplementedBy relationship (dual to IsImplementation) and make sure to handle that appropriately in the backend.

This also brings up the question of what to do in the remote-remote case. Let's handle that separately instead of in this issue. I've filed: #65