fix: fix resolve project local plugin's file path (#4095)

Thanks to @SneakyMax
See https://github.com/vuejs/vue-cli/issues/4014#issuecomment-497993827

(cherry picked from commit 78e1c4cf63)

packages/@vue/cli-service/lib/Service.js

@@ -189,7 +189,7 @@ module.exports = class Service {
       }
       plugins = plugins.concat(files.map(file => ({
         id: `local:${file}`,
-        apply: loadModule(file, this.pkgContext)
+        apply: loadModule(`./${file}`, this.pkgContext)
       })))
     }