add watchman to third-party, build ext_facts and ext_watchman by default (#8820)
Summary:
Unblocks external use of the native non-repo-auth autoloader
Pull Request resolved: https://github.com/facebook/hhvm/pull/8820
Test Plan:
- hhvm-mac and hhvm-cmake jobs
- HSL tests (use native autoloader) fail on OSS builds as watchman isn't configured, but show the extension is at least initializing correctly
```
fredemmott@fredemmott-fbmbp hsl % HHVM_BIN=$HOME/code/hhvm/build/hphp/hhvm/hhvm ./minitest.sh
Possible bad config node: Autoload.DBPath
Failed to update native autoloader, not natively autoloading /Users/fredemmott/code/hhvm/hphp/hsl. watchman::CommandValidationError: failed to validate command: resolve_projpath: None of the files listed in global config root_files are present in path `/Users/fredemmott/code/hhvm/hphp/hsl` or any of its parent directories. root_files is defined by the `/etc/watchman.json` config file and includes `.watchmanconfig`. One or more of these files must be present in order to allow a watch. Try pulling and checking out a newer version of the project?
ext_facts is required
```
Reviewed By: jthemphill
Differential Revision:
D27542496
fbshipit-source-id:
67054b0831dfd645d970f489f07ef51ed85fb47e