`folly/folly` used by Boost and others. The first directory serves as
an installation root of the library (with possible versioning a la
`folly-1.0/`), and the second is to distinguish the library when
-including files, e.g. `#include "folly/FBString.h"`.
+including files, e.g. `#include <folly/FBString.h>`.
The directory structure is flat (mimicking the namespace structure),
i.e. we don't have an elaborate directory hierarchy (it is possible