Correctly create visitorKeysMap

[zertosh]

visitorKeysMap has always been a copy of t.VISITOR_KEYS. After doing some archeology, it seems like there's been confusion wrt lodash's pick vs pickBy, which led to this.

When visitorKeysMap was first introduced in #109, it used lodash.pick@3. In that version, the predicate was called with (value, key, object). It was later switched to lodash.pickby@4 in #301. Because in lodash@4, pick switched to calling the predicate with (key, value, object), and pickby kept the old pick behavior. Throughout this, the value passed to indexOf was always an array (those are the values of t.VISITOR_KEYS), and indexOf was comparing that to strings (the values of t.FLIPPED_ALIAS_KEYS.Flow). So that meant that -1 === -1 and visitorKeysMap became a shallow copy of t.VISITOR_KEYS. #450 didn't help because it copied the same behavior.

This is a very big change. I ran this against the tests in https://github.com/gajus/eslint-plugin-flowtype and on a full eslint run of https://github.com/facebook/nuclide - and babel-eslint continues to work.

[zertosh]

Abandoning this since I have a PR coming up that removes all of this.