Cleanup usage of base PlexObject methods

[JonnyWong16]

Description

• Cleanup usage of findItems().
• Cleanup usage of listAttrs().
• Cleanup utils imports.

TODO: Rebase and update after #530.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated the docstring for new or existing methods