Fold Incremental External Dependency Nodes Into External Dependency Nodes

Remove this distinction without a difference. Originally, the thought
was to
1) Isolate the cross-module build infrastructure
2) Provide a signal to the driver that a dependency had swiftdeps info
   in it

But the driver need only notice swiftmodule files as external
dependencies and try to extract that information if it can to divine the
signal it needs. Additionally, we can give it fingerprints as priors to
let it know there might be incremental info to be had.

tools/swift-dependency-tool/swift-dependency-tool.cpp

@@ -109,8 +109,6 @@ void ScalarEnumerationTraits<swift::fine_grained_dependencies::NodeKind>::
   io.enumCase(value, "member", NodeKind::member);
   io.enumCase(value, "dynamicLookup", NodeKind::dynamicLookup);
   io.enumCase(value, "externalDepend", NodeKind::externalDepend);
-  io.enumCase(value, "incrementalExternalDepend",
-              NodeKind::incrementalExternalDepend);
   io.enumCase(value, "sourceFileProvide", NodeKind::sourceFileProvide);
 }