Really dogfood clippy
The dogfood success condition was inverted in `tests/dogfood.rs`:
```rust
assert!(!failed_packages.is_empty(), …);
```
while instead the `failed_packages` collection must be empty:
```rust
assert!(failed_packages.is_empty(), …);
```
And indeed, several clippy lint source files were not clean and had to be fixed in the process.
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6cebe58dfe