In the storage.ts class, our code to remove plugins (exploded directories and files) is using old-school unlink and remove with callbacks and the likes.

We should "promisify" it to make it more readable and consistent with the rest of the codebase using Promises.

See:

• the class to modify https://github.com/jenkins-infra/evergreen/blob/master/distribution/client/src/lib/storage.ts

Main pointers about NodeJS's util.promisify:

• http://2ality.com/2017/05/util-promisify.html
• http://2ality.com/2017/05/util-promisify.html