Rework publish-mpp-root-module-in-platform.gradle (#1020)
Rework publish-mpp-root-module-in-platform.gradle (#1020)

* Don't duplicate the platform artifact in the root module
* Instead of copying the platform POM to the root module, generate a
  new POM for the root module that shows that there's only a single dependency
  on the platform module (foo -> foo-jvm)

Fixes #996
Kotlin/kotlinx.serialization,Kotlin/kotlinx.serialization