`gen` is a reserved keyword in Rust 2024. This is a requirement to run
`cargo fmt` on the Rust 2024 edition.

Love this tooling! Small adjustment to help on error messaging 🙏 

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## Summary

<!-- What's the purpose of the change? What does it do, and why? -->

The previous message was missing the word **“like,”** which made it read
a tad awkwardly.
This PR inserts the missing word so the error reads naturally:
**After:**
```
$ uvx ch1.py 
error: It looks like you tried to run a Python script at `ch1.py`, which is not supported by `uvx`

hint: Use `uv run ch1.py` instead
```

**Before:**
```
$ uvx ch1.py 
error: It looks you tried to run a Python script at `ch1.py`, which is not supported by `uvx`

hint: Use `uv run ch1.py` instead
```

## Test Plan

<!-- How was it tested? -->
- `cargo run -- uvx examples/ch1.py` shows the updated message (see
“After” above).
- `cargo test` passes.

Some `ref`-removal and `use<>` fixes for the Rust Edition 2024
migration, which are also compatible with Rust Edition 2021.

We may run on case-sensitive file systems (Linux, generally) or on
case-insensitive file systems (Windows, generally), while modules in
Python may be lower or upper case. For robustness over filesystem
casing, we require an explicit module name for modules with upper cases.

Fixes #13419

I hit this error case with a broken wheel.

We broke the rkyv deserialization in
e70cf25#diff-348b24d7a84672ab2873833988156191995ff467619a77f548adbd9808549999L30-R41
by placing `.tar.bz2` in the position of `.tar.gz`. We need to
invalidate the cache bucket.

Fixes #13492

Rustfmt introduces a lot of formatting changes in the 2024 edition. To
not break everything all at once, we split out the set of formatting
changes compatible with both the 2021 and 2024 edition by first
formatting with the 2024 style, and then again with the currently used
2021 style.

Notable changes are the formatting of derive macro attributes and lines
with overly long strings and adding trailing semicolons after statements
consistently.