[electrum] import the slp module directly

Summary:
instead of importing from the slp package via __init__.py

In a following commit I plan to rename the package tokens, so we can add more token related modules (empp.py, alp.py...) to it.

Test Plan: run tests and run the application

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17757