overhaul the duplicate class checker

[plaird]

This is a big refactor/overhaul of the duplicate class checker feature. Changes:

• the feature is now entirely written in python; previously is was half done in bash (to create the list of .class files found in all inner jars) and half done in python (to look for dupes)
• springboot rule user can now provide the allowlist file, which tells the dupe checker to ignore dupes from the allow list jars
• documentation improvements

This fixes #29