Update the jar `Handler` class to support a non-reflective fallback
mechanism when possible. The updated code attempts to capture a regular
jar URL before our handler is installed. It can then use that URL as
context when creating the a fallback URL. The JDK jar `Handler` will
be copied from the context URL to the fallback URL.

Without this commit, resolving new Tomcat URLs of the form
`jar:war:file:...` would result in an ugly "Illegal reflective access"
warning.

Fixes gh-18631