Add fat jars for gradle plugin (#2149)

* [gradle-plugin] Bundle big Kotlin plugin into the native plugin jar

* Use big Kotlin's version in the Gradle plugin

* [gradle-plugin] Exclude big Kotlin plugins from final jar

* Use composite build again

* Remove MULTIPLATFORM.md

* [gradle-plugin] Fix adding common sources

* [gradle-plugin] Fix test running

_nav_reference.yml

@@ -14,10 +14,6 @@
   url: platform_libs.html
   title: "Platform Libraries"
 
-- md: MULTIPLATFORM.md
-  url: multiplatform.html
-  title: "Multiplatform Projects"
-
 - md: INTEROP.md
   url: c_interop.html
   title: "C Interop"