Refactor WatchmanAutoloadMap into FactsStoreImpl
Summary:
Facts has historically depended on Watchman to track file changes. Open-source users may not have access to Watchman, and we do need everyone in open-source to get onto native autoloading so we can remove `require_once` from the Hack language.
This diff renames `WatchmanAutoloadMap` to `FactsStoreImpl`, moving Watchman details out of the header. Really, this moves pretty much everything out of the header.
The next diff factors Watchman functionality into one implementation of a `Watcher` interface, which may use Watchman or may not.
Reviewed By: fredemmott
Differential Revision:
D29018377
fbshipit-source-id:
b4b1369d0f5875d3349df1930c876208f88f01e4