rename LanguageService.getSourceFile to LanguageService.getNonBoundSourceFile and mark it as internal

src/server/client.ts

@@ -613,7 +613,7 @@ namespace ts.server {
             throw new Error("SourceFile objects are not serializable through the server protocol.");
         }
 
-        getSourceFile(fileName: string): SourceFile {
+        getNonBoundSourceFile(fileName: string): SourceFile {
             throw new Error("SourceFile objects are not serializable through the server protocol.");
         }
 

src/server/editorServices.ts

@@ -1330,7 +1330,7 @@ namespace ts.server {
         }
 
         isExternalModule(filename: string): boolean {
-            const sourceFile = this.languageService.getSourceFile(filename);
+            const sourceFile = this.languageService.getNonBoundSourceFile(filename);
             return ts.isExternalModule(sourceFile);
         }