Previously, TomcatEmbeddedWebappClassLoader would find resources
in WEB-INF/classes. However, unlike standalone Tomcat, we know that in
a Boot app WEB-INF/classes is on the class path of the parent class
loader so the resources will be found when the parent is queried
(which happens as part of the normal search algortithm for both
getResource(String) and getResources(String)).

This commit overrides findResource(String) and findResources(String)
to return null and an empty enumeration respectively. This prevents
TomcatEmbeddedWebappClassLoader from finding resources in
WEB-INF/classes and returning war: URLs for them that duplicate the
jar: URLs that will be found when the parent is queried.

Closes gh-9014