Change Node API to represent package as an object instead of string

[bmish]

https://github.com/bmish/check-dependency-version-consistency#node-api

Before:

const result = {
  isFixable: true,
  isMismatching: true,
  name: 'eslint',
  versions: [
    {
      packages: ['package1', 'package2'],
      version: '^7.0.0',
    },
    {
      packages: ['package3'],
      version: '^8.0.0',
    },
  ],
};

After:

// ...
packages: [{ pathRelative: 'package1' }, { pathRelative: 'package2' }],

This is for better extensibility, so we can add other info about each package later. Including whether a dependency is a devDep/dep/resolution/peerDep/etc in each package.