Use explicit extensions for imports within src (#58421)

src/tsc/_namespaces/ts.ts

@@ -1,3 +1,3 @@
 /* Generated file to emulate the ts namespace. */
 
-export * from "../../compiler/_namespaces/ts";
+export * from "../../compiler/_namespaces/ts.js";

src/tsc/tsc.ts

@@ -1,4 +1,4 @@
-import * as ts from "./_namespaces/ts";
+import * as ts from "./_namespaces/ts.js";
 
 // This file actually uses arguments passed on commandline and executes it