[Gradle][KPM] Split fragment factories into composable entities

This compasability will ensure that external target authors
(AGP) are able to reuse most of KPM code whilst allowing a lot of
customizability.

libraries/tools/kotlin-project-model/src/main/kotlin/KotlinModuleFragment.kt

@@ -21,6 +21,8 @@ interface KotlinModuleFragment {
 
     // TODO: should this be source roots or source files?
     val kotlinSourceRoots: Iterable<File>
+
+    companion object
 }
 
 interface KotlinModuleVariant : KotlinModuleFragment {
@@ -72,4 +74,4 @@ class BasicKotlinModuleVariant(
 ), KotlinModuleVariant {
     override val variantAttributes: MutableMap<KotlinAttributeKey, String> = mutableMapOf()
     override fun toString(): String = "variant $fragmentName"
-}
+}