[ExplicitModule] Teach scanner emit loaded module trace

During explicit module build, teach dependency scanner to emit the
module trace file instead of each following compile job command. This
reduces the duplicated info, and allows supporting fully cached build
that only loads module from CAS thus cannot produce the path to the
original module file on disk.

rdar://170007480

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -785,6 +785,20 @@ ModuleDependencyScanner::getMainModuleDependencyInfo(ModuleDecl *mainModule) {
     mainDependencies.updateCommandLine(buildArgs);
   }
 
+  // Dependency only imports
+  {
+    for (auto &m : ScanASTContext.SearchPathOpts.DependencyOnlyModuleImports) {
+      // If module is seen, then it is not dependency only, ignore.
+      if (alreadyAddedModules.contains(m))
+        continue;
+
+      mainDependencies.addModuleImport(
+          m,
+          /*isExported=*/false, AccessLevel::Public, &alreadyAddedModules);
+      mainDependencies.addDependencyOnlyImport(m);
+    }
+  }
+
   return mainDependencies;
 }