[Dependency Scanning] Model main module as separate dependency kind: SwiftSource

These kinds of modules differ from `SwiftTextual` modules in that they do not have an interface and have source-files.
It is cleaner to enforce this distinction with types, instead of checking for interface optionality everywhere.

include/swift/DependencyScan/DependencyScanImpl.h

@@ -37,7 +37,8 @@ struct swiftscan_dependency_info_s {
   /// The format is:
   /// `<module-kind>:<module-name>`
   /// where `module-kind` is one of:
-  /// "swiftTextual"
+  /// "swiftInterface"
+  /// "swiftSource"
   /// "swiftBinary"
   /// "swiftPlaceholder"
   /// "clang""