Remove the embedded-kotlin-repository It was based on ClientModule dependencies that caused more troubles than anything. The main one being that the artificial dependency graph needed to be kept up to date with each Kotlin version. Moreover since several Kotlin versions, declaring a repository is necessary in any case. This change simplifies the code and should make the runtime a bit faster.
However, the Kotlin dependencies when applying the `kotlin-dsl` plugin are now required to be downloaded. This isn't a big change and should only impact people using the `kotlin-dsl` plugin without depending on Kotlin in other parts of their build. This could be alleviated in the future if dependency resolution considers the Gradle install/distro as a source of artifacts in the same way it uses maven local.
Along the way, the pinning of Kotlin dependencies to the embedded version for the build scripts classpath has been moved from a resolution rule to proper, faster, dependency constraints.