Add a concept of a concrete-library which has access to Bukkit but is not a plugin.

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

@@ -1,3 +1,7 @@
 plugins {
   id("lgbt.mystic.foundation.concrete-plugin")
 }
+
+dependencies {
+  implementation(project(":bukkit-plugins:common-library"))
+}