return lookupCache.Do(lookupCacheKey{proxy, path}, func() Repo {
		return newCachingRepo(ctx, path, func(ctx context.Context) (Repo, error) {
			r, err := lookup(ctx, proxy, path)
			if err == nil && traceRepo {
				r = newLoggingRepo(r)
			}
			return r, err
		})
	})
}

// lookup returns the module with the given module path.
func lookup(ctx context.Context, proxy, path string) (r Repo, err error) {