`wit-parser`: Expose source location information for custom validation

[wilfreddenton]

Currently Spans are private and the location information in UnresolvedPackage is "consumed" when resolved. Tools that add additional validation/linting have no way to access this location information.

[wilfreddenton]

With #2419 merged the bulk of this work is complete; however, there are a couple tasks remaining.

1. Eliminating duplication of Span information in UnresolvedPackage
2. Adding spans to function parameters

I will complete these in separate PRs which I should be able to work on soon.