Scans the web application's sub-directory identified by startPath,
along with its sub-directories, for TLDs.

Initially, rootPath equals /WEB-INF/. The /WEB-INF/classes and
/WEB-INF/lib sub-directories are excluded from the search, as per the
JSP 2.0 spec.

Keep in sync with o.a.j.comiler.TldLocationsCache