Improve scenario update script (#759)

Following #757, improves the script for generating scenario test cases
with:

- A requirements file
- Support for downloading packse scenarios from GitHub dynamically
- Running rustfmt on the generated test file
- Updating snapshots / running tests

crates/puffin-cli/tests/pip_install_scenarios.rs

@@ -1,6 +1,6 @@
 #![cfg(all(feature = "python", feature = "pypi"))]
 
-/// Generated by `scripts/scenarios/generate.py`
+/// Generated by `./scripts/scenarios/update.py`
 use std::process::Command;
 
 use anyhow::Result;