Some examples of the usage:

• Use BuildServiceProvider or some map chain involving it as a parameter value for some ValueSource and obtain the value at configuration time.
• Use some mapped BuildServiceProvider as value for an @Input-annotated property of a task, and run the task as part of buildSrc build or some included build that contributes a plugin.

In #22337 we added a clearer error message for these usages, but it introduces unpleasant asymmetry and leaves a gap in the API: there's no way to share some resource among ValueSources and other configuration-time clients (see the linked issue for more elaborate use case description). Maybe we should make it work.