Use case persistify + circular json

See https://github.com/royriojas/flat-cache/issues/5

To demonstrate the issue:

Run npm run bundle-watch (or persistify ./index.js --cache-dir ./cache --watch -o bundle.js)

Leads to:

TypeError: Converting circular structure to JSON

Reasons:

• The issue appear when a foo package use a browserify.transform field in the package.json file (with an options object)

  (...)
  "browserify": {
    "transform": [
      [ "./transform", { } ]
    ]
  }
  

• The options object will be populated with a circular reference to the main browserify packageCache object when used with watchify (hence the --watch option)

  # with `var cache` the persistify cache:
  cache.persistifyArgs.packageCache['<absolute path to>/foo/package.json'].browserify.transform[0][1]._flags.packageCache === cache.persistifyArgs.packageCache