rearrange build system plugin API to move library finding to the cli

`Module.getLibraryFor` throws an exception if no compatible library is found, which so far has been fine because that's a case where we want to exit anyway. We're changing that, which makes it awkward that the build system plugins are the callers of that API because the decision of how to handle modules with no compatible library is their decision, and that behavior really isn't something that should vary by the user's choice of build system.

I think the better way to arrange this would be to give the plugin the already resolved `PrebuiltLibrary` objects which are resolved by the CLI before the build plugin is invoked.