[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.

lib/DependencyScan/StringUtils.cpp

@@ -55,6 +55,12 @@ swiftscan_string_set_t *create_set(int count, const char **strings) {
   return set;
 }
 
+swiftscan_string_set_t *create_empty_set() {
+  swiftscan_string_set_t *set = new swiftscan_string_set_t;
+  set->count = 0;
+  return set;
+}
+
 const char *get_C_string(swiftscan_string_ref_t string) {
   return static_cast<const char *>(string.data);
 }