Fix https://issues.apache.org/bugzilla/show_bug.cgi?id=52323
Do not download Cobertura, unless it has been enabled.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1538798 13f79535-47bb-0310-9956-ffa450edef68
Followup to r1538084
Add a comment on the new class. (Copied the text from the commit message).

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1538803 13f79535-47bb-0310-9956-ffa450edef68
First pass at switching to Jar abstract from Jar scanning code that
handles accessing resources in JARs in packed WARs.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1538805 13f79535-47bb-0310-9956-ffa450edef68