What problem does this feature solve?

We config mutil projects in vue/cli3，it'lll load all projects we configs. It waste time and most of the projects is useless.
For example,we have projceA,projectB,projectC,we only need to develop projectB,but vue/cli3 load all .
So,what the feature we need?That is,we use webpack-dev-serve listen to the url the browser accept.And then,resolve the projcet from url,then build the project we accept.
This feature improve development efficiency deeply.

What does the proposed API look like?

At this file,https://github.com/vuejs/vue-cli/blob/dev/packages/%40vue/cli-service/lib/commands/serve.js.

• We use webpack-dev-serve to get the project from url.

          server.app.get('/*.html', function (req, res, next) {})

• match the project

         const matched = req.originalUrl.match(/\/(.*)\.html/);

• dynamic load the entry use MultiEntryPlugin

const handleMultiEntry = (app, paths, compiler) => {
  paths = hotEntry.concat(paths);
  const multiEntry = new MultiEntryPlugin(process.cwd(), paths, app);
  multiEntry.apply(compiler);
  findHtmlPlugin(app).apply(compiler);
};

findHtmlPlugin means to load HTMLWebpackPlugin to this project

• recomplie the project

server.middleware.invalidate();

• return the build result to browser

server.middleware.waitUntilValid(() => {
    compiler.outputFileSystem.readFile(
      path.join(compiler.outputPath, req.originalUrl),
      function (err, result) {
        if (err) {
          return next(err);
        }
     
        res.set('content-type', 'text/html');
        res.send(result);
        res.end();
      }
    );
  });

That's all the step i describe about the feature.

I had implemented the feature in my local environment. works well.
I hope vue/cli team can accept this feature.Thanks for reading.
(pardon my poor description)