5e0ba6ea added support for reading a jar manifest's Class-Path
attribute when resolving changeable URLs from a URLClassLoader,
however it did not handle relative URLs, i.e. URLs without a protocol,
correctly.

This commit updates ChangeableUrls so that it uses the URL of the
JAR that contains the manifest as the base for any new URLs that
are created. When the Class-Path entry is relative, this base will
be used. When the Class-Path entry is absolutee, URL's constructor
will ignore the supplied base.

Closes gh-5665