[libomptarget] Move header files and CMake library definition

Future patches will add (private) header files in src/ that should
not be visible to plugins, so move the "public" ones to a new
include/ directory. This is still internal in a sense that the
contained files won't be installed for the user.
Similarly, the target agnostic offloading library should be built
directly in src/. The parent directory is responsible for finding
dependencies and including all subdirectories.

Differential Revision: https://reviews.llvm.org/D40797

llvm-svn: 319968