Enforce that all packages conform to Sonatype requirements

[jvican]

Sonatype fails to publish projects that do not have the right metadata. The required metadata is explained here: https://blog.idrsolutions.com/2015/06/how-to-upload-your-java-artifact-to-maven-central/.

We do check some of these parameters (like the license), but we should provide smarter checks to ensure that users do define these keys in their builds! This is a nice improvement that can be implemented in the ReleaseEarlyPlugin class and tested via scripted (by getting a correct build that publishes and removing iteratively requirements that should be detected by the plugin).

Ideally, we would like to warn our users when they start up sbt so that they can quickly define these settings if they expect releasing to Sonatype to work.