MDL-90 lib/setup: extend include_path to cover the PEAR libs we distribute

MDL-90 lib/setup: extend include_path to cover the PEAR libs we distribute

Almost no cost - most of our require/include calls are with
absolute paths, which will skip the include_path. The few
that are not absolute are usually to include PEAR libs -
so this is appropriate and avoids having to change the
PEAR libs themselves when they make references to other
PEAR libs.

Using the include path does mean that OS-installed PEAR libs
will override ours. There's a small compat risk here - if you
need to ensure a particular version, use an absolute path.