Support for multiple package.json inputs

[aleclarson]

Sometimes dev dependencies can exist in a monorepo's root package.json, or maybe I have multiple entries from different packages that I want merged. In the latter case, it would probably be best to add support for an entry-specific config, like so:

// This maps "outSrcRoot" => "entry"
entries: {
  'src/foo': 'packages/foo/src/index.ts',
  'src/bar': 'packages/bar/src/index.ts',
}

Then let's add an array version of pkgRoot option. The highest common ancestor is used to compute module IDs.

pkgRoots: ['/path/to/packages/foo', '/path/to/packages/bar']