.hide MultiJvm configuration

[eed3si9n]

Akka's build contains a lot settings scoped to MultiJvm configuration, like formatting and header, but most subproject actually does not declare MultiJvm to be a configuration. When Akka upgrades to sbt 1.2.0, it will see a wall of warnings:

[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-actor-typed") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-actor-typed-tests") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-stream-tests-tck") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-osgi") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-protobuf") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
[warn] The project ProjectRef(uri("file:/Users/eed3si9n/work/akka/"), "akka-actor-testkit-typed") references an unknown configuration "multi-jvm" and was guessed to be "Multi-jvm".
[warn] This configuration should be explicitly added to the project.
...

One way to prevent this is to inject the configuration to all subprojects via a local AutoPlugin, but to make sure that the configuration does not leak out to POM etc, it should probably declare itself as private by calling .hide.