fix: use local gts path for generated tsconfig.json

Currently the global gts path is used when `gts init` is done using a
globally installed `gts`, which is a usual workflow.

Author: jinwoo

src/init.ts

@@ -152,13 +152,8 @@ async function generateTsConfig(options: Options): Promise<void> {
     }
   }
 
-  // typescript expects relative paths to start with './'.
-  const baseConfig = './' +
-      path.relative(
-          options.targetRootDir,
-          path.resolve(options.gtsRootDir, 'tsconfig-google.json'));
   const tsconfig = formatJson({
-    extends: baseConfig,
+    extends: './node_modules/gts/tsconfig-google.json',
     compilerOptions: {rootDir: '.', outDir: 'build'},
     include: ['src/*.ts', 'src/**/*.ts', 'test/*.ts', 'test/**/*.ts'],
     exclude: ['node_modules']

test/test-kitchen.ts

@@ -85,8 +85,13 @@ test.serial('init', async t => {
         `npx -p ${stagingPath}/gts.tgz --ignore-existing gts init -n`,
         execOpts);
   }
-  fs.accessSync(`${stagingPath}/kitchen/tsconfig.json`);
-  t.pass();
+  const tsconfigPath = `${stagingPath}/kitchen/tsconfig.json`;
+  fs.accessSync(tsconfigPath);
+
+  // The `extends` field must use the local gts path.
+  const tsconfigJson = fs.readFileSync(tsconfigPath, 'utf8');
+  const tsconfig = JSON.parse(tsconfigJson);
+  t.deepEqual(tsconfig.extends, './node_modules/gts/tsconfig-google.json');
 });
 
 test.serial('use as a non-locally installed module', async t => {