Mark the internal headers as private for IWYU

The installed headers below ceres/internal are implementation details,
but nothing tells include-what-you-use and similar tools to keep users
from including them. Annotate them, including the generated
configuration and export headers, with IWYU pragmas that mark them as
private and map the Euler angle conventions and Jet traits to their
public headers.

Change-Id: I7ab3c59450b6e88f938d7f54a9bec5b4dbb9cc64