* refactor(sqlx-cli): Try avoiding a full clean with `--merged`
* docs(sqlx-cli): Sprinkle some comments on the metadata changes
* refactor(sqlx-cli): Make the new recompiltion setup unit-testable
* fix(sqlx-cli): Only pass in `$RUSTFLAGS` when set when using `--merged`
* refactor(sqlx-cli): `cargo clean -p` works by name so rip out pkgid code
* chore(sqlx-cli): Remove unused imports
