perf: improve getTemplate() when cache is enabled

Checking for file existence is unnecessary when cache contains entry.

benchmark/index.ts

@@ -1,11 +1,13 @@
 import output from './output'
 import tag from './tag'
 import demo from './demo'
+import layout from './layout'
 
 async function main () {
   await output()
   await tag()
   await demo()
+  await layout()
 }
 
 main()