Fix some cross-module definition calls

[chrismwendt]

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Before:

After:

Admittedly, this doesn't work in all cases (e.g. the call to SourceKit.CodeComplete in https://github.com/RLovelett/langserver-swift/issues/52), which is pretty ironic because that's the case that I tried to fix 😒

If anyone has any tips for figuring out what arguments to pass to swift to get it to resolve these properly, that would be very helpful, because so far my best strategy is Try Random Combinations And Hope It Works ™️

[chrismwendt]

I probably don't actually understand this much more than you do. My guess is that providing more .swift files to the compiler helps it find definitions of classes/structs/protocols.