How to properly attach a library to CheckoutPaneForm
As usual, I was messing something up.  JS libraries need : {}.  Updating commerce_paypal.libraries.yml to this solves it:

paypal.checkout:
  remote: https://www.paypalobjects.com/api/checkout.js
  license:
    name: MIT
    gpl-compatible: true
  js:
    https://www.paypalobjects.com/api/checkout.js: {}
express-checkout:
  js:
    js/paypal_ec.js: {}
  dependencies:
    - commerce_paypal/paypal.checkout