Convert the C and C++ files in cdatapack, clib, cstore, and ctreemanifest
to always include files from the root of fb-hgext. This simplifies the build
process by no long requiring a variety of separate include directories to be
specified on the compiler command line.
This will also make it easier to re-use these extensions in other projects
with different build systems.
This corresponds to the Facebook diff D5588676.