Add plugin variants

Change configuration adding Gradle plugin variants. 'main' SourceSet
is used for minimal supported Gradle versions, 'common' SourceSet for
common code that is similar between versions, plus, currently,
'gradle70' SourceSet for Gradle 7.0+ versions.

^KT-49227 In Progress

build.gradle.kts

@@ -328,7 +328,8 @@ extra["compilerArtifactsForIde"] = listOf(
 
 // TODO: fix remaining warnings and remove this property.
 extra["tasksWithWarnings"] = listOf(
-    ":kotlin-gradle-plugin:compileKotlin"
+    ":kotlin-gradle-plugin:compileKotlin",
+    ":kotlin-gradle-plugin:compileGradle70Kotlin"
 )
 
 val tasksWithWarnings: List<String> by extra