move watchman includes into their own directory

Summary:
Bring Watchman closer to the EdenFS file name convention by moving
source files and includes into watchman/.

Reviewed By: fanzeyi

Differential Revision: D29242789

fbshipit-source-id: 6e29a4a50e7202dbf6b603ccc7e4c8184afeb115

build/fbcode_builder/manifests/watchman

@@ -23,7 +23,8 @@ googletest
 rust
 
 [shipit.pathmap]
-fbcode/watchman = .
+fbcode/watchman = watchman
+fbcode/watchman/oss = .
 fbcode/eden/fs = eden/fs
 
 [shipit.strip]