[makeinstancesufo] keep all public lib keys in instances

[arialcrime]

Description

This slightly alters the behaviour of makeinstancesufo’s postProcessInstance. clearCustomLibs now keeps all lib keys that start with public..

fixes adobe-type-tools/afdko#1746

Checklist:

• [x] I have followed the Contribution Guidelines
• [ ] I have added test code and data to prove that my code functions correctly
• [ ] I have verified that new and existing tests pass locally with my changes
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation

[arialcrime]

Not very experienced with these tests, but it seems like the hiccup in “C and Python coverage / coverage (pull_request)” is not related to my changes, right?