When a repository rule is fetch attributes are iterated over in `enforceLabelAttributes` to prepopulate the environment, restarting the fetch each time a new dependency is discovered.

This is faster than calling `repository_ctx.path(...)` early in the repository rule implementation function but still has considerable overhead.

This PR defers throwing `NeedsSkyframeRestartException` till after every attribute has been processed, greatly reducing the number of restarts and iterations.

In an internal project these optimisations are particularly noticeable.
Before: 2min 8s, 2min 7s
After: 1min 35s, 1min 27s

Closes bazelbuild#20434.

PiperOrigin-RevId: 588090528
Change-Id: I7917b137d6e60b6d6a73189cf396418a85b3ec28