Fill in missing fields when retrieving providers

Adding a field to a provider is a breaking change for rules intended to be compatible with rules_go. However, we need to do this frequently when adding new features and fixing bugs.

Adding a field to a struct in Go is much less disruptive, since zero values are usually meaningful and useful. Starlark doesn't have that, but we could access provider fields with `getattr` and a default value.

A better option might be to access providers through getter functions that rebuild providers when fields are missing. We already have some of these functions like `get_archive` to deal with the aspect. Those functions could check if any field is missing, and create a new provider object with default values for those fields if so.

This would be a frequent operation, so if this has a measurable analysis performance impact, it may not be worth doing.

Start by reading the existing get_archive function and other provider getter functions mentioned in the issue. Define how missing provider fields should receive defaults while preserving rules_go compatibility, then measure analysis performance to determine whether the approach is acceptable.

Written by the indexing model from the issue text.