Structured and self-contained prepare

[ulivz]

Summary

In order to write unit tests more conveniently, and have a clearer context to dig in the development of plugin API, the PR is to adjust the prepare from single file to the following directory structure:

└─── prepare
      ├── codegen.js
      ├── index.js
      ├── loadConfig.js 
      ├── resolveOptions.js 
      └── util.js

• loadConfig.js: migrate from lib/util/loadConfig.js, lib/util will only contain the shared utilities.
• util.js: methods in this util will only be used in the prepare process.

This PR also complements the UT of the prepare process's core (resolveOptions & core utilities).

@yyx990803, Hey, Evan, could you have a look at it before I merge it? Thanks!

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• Feature
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