[ParseableInterface] Distinguish SDK and non-SDK dependencies

This allows the SDK to be relocated without automatically resulting in a
rebuild.

Based on an old patch from Jordan Rose.

test/ParseableInterface/ModuleCache/Inputs/mock-sdk/SdkLib.swiftinterface

@@ -0,0 +1,8 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -parse-stdlib -module-name SdkLib
+
+@_exported import ExportedLib
+
+public var testValue2: ExportedInterface {
+  @inlinable get { return ExportedInterface() }
+}