Make `db.package_by_name()` and `db.root_by_package()` backed by Salsa queries

[DavisVaughan]

See #1287 (comment)

I think we started with file_by_path(), which can't be a query because FilePath is not a Salsa key, but that reason doesn't apply to package-by-name (if we make it take a Name instead of &str) and root-by-package.

Idea is to make package_by_name_query() and root_by_package_query() that underly these into queries