Re-organize sources

Re-organize sources

 * Create a top-level 'src' directory.
 * Move all private headers from the public include directory to the
   'src' directory.
 * Rename all those private headers to remove the 'internal' notation.
 * Use double quotes in `#include` directives when including a private
   header so that we can easily know.

Change-Id: I05fbb81d969b3735aaf303ec2c222be7142c19ab
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1413
Tested-by: jenkins
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>