fix: fix ESDIR errors when outputDir contains dots

closes #2414

When `toType` is not specified, copy-webpack-plugin will infer it from
the value of `to`. So if it contains one or more dots, it would be
treated as a file with extension.

Author: haoqunjiang

packages/@vue/cli-service/lib/config/app.js

@@ -262,6 +262,7 @@ module.exports = (api, options) => {
           .use(require('copy-webpack-plugin'), [[{
             from: publicDir,
             to: outputDir,
+            toType: 'dir',
             ignore: publicCopyIgnore
           }]])
     }