Implement support for custom concrete items.

samples/mixed/bukkit-plugins/build.gradle.kts

@@ -2,9 +2,10 @@ plugins {
   id("gay.pizza.foundation.concrete-root")
 }
 
-concrete {
+concreteRoot {
   minecraftServerPath.set("server")
   paperServerVersionGroup.set("1.19")
   paperApiVersion.set("1.19.3-R0.1-SNAPSHOT")
   acceptServerEula.set(true)
+  expansiveItemInclusion.set(true)
 }

samples/mixed/bukkit-plugins/hello-world/build.gradle.kts

@@ -7,6 +7,6 @@ dependencies {
   implementation(project(":bukkit-plugins:common-library"))
 }
 
-plugin {
+concreteItem {
   dependency(project(":bukkit-plugins:goodbye-world"))
 }

samples/mixed/other-library/build.gradle.kts

@@ -1,3 +1,10 @@
 plugins {
   id("gay.pizza.foundation.concrete-base")
 }
+
+concreteItem {
+  type.set("library")
+  fileInclusion {
+    tasks.jar.get().outputs.files.associateWith { "library-jar" }
+  }
+}