Fix reference pass error in package config

[PeachScript]

Type

Bugfix

Tests

All tests are passing

Description

We will get an error config if we loading config multiple times from package.json, because the config is passed by reference.

For example, we use postcss and add a postcss field into package.json like this:

...
"postcss": {
  "plugins": {
    "autoprefixer": {
      "grid": true
    }
  }
}
...

Then the first time in ./src/transforms/postcss.js:L34 we can get a correct config like above.

But if we change some styles, in the second time we will get an error config like this:

"postcss": {
  "plugins": [null, null, null],
  "from": "...",
  "to": "..."
}

All plugins will missing because it has been overwritten in ./src/transforms/postcss.js:L48, then compiling will break.