Fix deprecation warnig in `packing.py` and replace `.format()` with f-strings where possible.

[chrisjonesBSU]

PR Summary:

This is a PR that does 2 things:

1) Fix a deprecation warning that is being raised about find_executable in distutils

>>> from distutils.spawn import find_executable
>>> find_executable("PACKMOL")
<stdin>:1: DeprecationWarning: Use shutil.which instead of find_executable

2) It also replaces .format() with f-strings in many places in an effort to clean things up a bit (I was bored at an airport)

PR Checklist

---

• [ ] Includes appropriate unit test(s)
• [ ] Appropriate docstring(s) are added/updated
• [ ] Code is (approximately) PEP8 compliant
• [ ] Issue(s) raised/addressed?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 87.34%. Comparing base (8649337) to head (e120962). Report is 39 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1192 +/- ## ========================================== - Coverage 87.35% 87.34% -0.01% ========================================== Files 62 62 Lines 6586 6584 -2 ========================================== - Hits 5753 5751 -2 Misses 833 833 ```

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[chrisjonesBSU]

Merging this after discussing with Cal on a mosdef dev call.