build: use compileOnlyApi to expose AST transforms to downstream compileClasspath

Switch from `compileOnly` to `compileOnlyApi` to ensure that the `DelegateAsync` AST transformation classes are available
on the downstream `compileClasspath`.

Author: matrei

grails-datastore-gorm-async/build.gradle

@@ -29,13 +29,14 @@ dependencies {
         // impl: Promises
     }
 
-    compileOnly project(':grails-datastore-async'), {
-        // comp: DelegateAsync
+    compileOnlyApi project(':grails-datastore-async'), {
+        // AST (source retention annotation): DelegateAsync
         if (excludeUnusedTransDeps) {
             // API dependencies in grails-datastore-async
             exclude group: 'org.grails', module: 'grails-async'
         }
     }
+
     compileOnly 'org.apache.groovy:groovy'
 }