Previously the Kotlin/Native version was hardcoded in sources of the
Gradle plugin. Such an approach is inconvenient when we want to
build Kotlin with a custom K/N version but without changes in sources
(e.g. during CI daily runs).
This patch adds a project property `versions.kotlin-native` which can
be set during build to override Kotlin/Native version. Also this
patch gets rid of hardcoding this version in sources and stores it in
a properties file packed with the Gradle plugin in a jar.
Ilya Matveev
acf8a0454f