Support for base projects which do not contain the Bukkit API.

Also fixes a bug where root and plugin couldn't exist in the same project.

samples/simple/build.gradle.kts

@@ -6,4 +6,5 @@ concrete {
   minecraftServerPath.set("server")
   paperVersionGroup.set("1.19")
   paperApiVersion.set("1.19-R0.1-SNAPSHOT")
+  acceptServerEula.set(true)
 }