[Groovy3] Simplify Groovy 2/3 compatibility layer

settings.gradle

@@ -8,11 +8,10 @@ include "spock-guice"
 include "spock-junit4"
 include "spock-temp"
 include "spock-testkit"
-//Remove once Groovy 2.5 support is dropped
-include "groovy2-compat"
 
 if ((System.getProperty("variant") as BigDecimal ?: 3.0) != 3.0) {
-  include "internal-backport"
+  //Remove once Groovy 2.5 support is dropped
+  include "spock-groovy2-compat"
 }
 
 // https://issues.apache.org/jira/projects/TAP5/issues/TAP5-2588

spock-core/core.gradle

@@ -23,7 +23,11 @@ dependencies {
   compile libs.cglib, optional
   compile libs.objenesis, optional
 
-  compile project(':groovy2-compat')
+  if (variant == 2.5) {
+    dependencies {
+      compile project(':spock-groovy2-compat')
+    }
+  }
 
   coreConsoleRuntime groovyConsoleExtraDependency
 }

spock-groovy2-compat/groovy2-compat.gradle

@@ -0,0 +1,27 @@
+//Module with compatibility layer to keep codebase compilable with both Groovy 2.5 and 3.0
+//
+//WARNING. Do not mix groovy:3.x and spock-groovy2-compat:2.x-groovy-2.5 jars on classpath!
+
+apply from: script("publishMaven")
+
+ext.displayName = "Spock Framework - Groovy 2 Compatibility Module"
+
+description = "Spock's Groovy 2 Compatibility Module provides support for using Spock 2 with Groovy 2.5."
+
+jar {
+  manifest {
+    attributes(
+      'Created-By': "${System.properties['java.version']} (${System.properties['java.vendor']} ${System.properties['java.vm.version']})".toString(),
+      'Build-Date': buildDate,
+      'Build-Time': buildTime,
+      'Build-Revision': versioning.info.commit,
+      'Specification-Title': project.name,
+      'Specification-Version': baseVersion,
+      'Specification-Vendor': 'spockframework.org',
+      'Implementation-Title': project.name,
+      'Implementation-Version': variantLessVersion,
+      'Implementation-Vendor': 'spockframework.org',
+      'Automatic-Module-Name': 'org.spockframework.spring'
+    )
+  }
+}