Add namespace qualified Ceres::ceres CMake target

- This reflects modern CMake style, and also provides a measure of
  protection against missing find_package() imports in downstream
  clients resulting in linker errors when 'ceres' matches the compiled
  library and not the imported target.
- The original 'ceres' target remains, as a local imported interface
  target created by CeresConfig for backwards compatibility.

Change-Id: Ie9ed8de9b7059bc0cae1ae5002bb94d8fe617188