[tx] Replace UFO lib.plist parsing with libxml2

[kaydeearts]

Description

This PR replaces lib.plist parsing code with libxml2 #1520 . (part of the tx parser replacement with libxml2 project), and is related to #1515

This should also fix #1306 , #1130 , #1305 , which discuss that tx/makeotf should not fail when missing lib.plist.
A missing lib.plist should now emit a warning to the user instead of failing. These warnings have been improved and tests have been added to both tx_test.py and makeotf_test.py.
The only exception to this is for cidkeyed UFOs that have cid identifiers defined within the glyphs files, for which there was a failure message already existing. This test case has also been added.

Checklist:

• 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
• 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

[lgtm-com]

This pull request introduces 1 alert when merging b1e4b28 into b02b850 - view on LGTM.com

new alerts:

• 1 for Wrong type of arguments to formatting function

[skef]

Looks good to me!

[josh-hadley]

👍