To allow Policy Packs to run against Pulumi programs written in all languages, we now look for the `@pulumi/pulumi/cmd/run-policy-pack` module in the Policy Pack's node_modules (instead of in the Pulumi program's node_modules; which doesn't exist for non-node languages). The `@pulumi/policy` library that a Policy Pack will depend on should already depend on a recent enough version of `@pulumi/pulumi`. When we can't find the module, it's more likely it's due to the dependencies for the Policy Pack not being installed. Provide a more helpful error message in this case.