Fix maven dependency scope conflicts in MergePlugin

Spring Framework's build is using a custom MergePlugin in order to merge
a project into another one and share/override configuration and
dependencies.

Prior to this commit, two projects merged into a third one could trigger
dependency conflicts when exporting the project definition into a POM.
When trying to define the scope for a given dependency, those two
projects would compete for this with the same priority, resulting in a
Gradle exception.

One could trigger this issue by running:

    ./gradlew :spring-orm:install -x javadoc

Because `spring-orm-hibernate4` and `spring-orm-hibernate5` get merged
into `spring-orm` and both define optional/test dependencies to
spring-mvc, etc.

This commit makes sure that two projects, when defining dependency
scopes, don't use the same priority value; the MergePlugin now adds the
index of the current subproject to the priority value. So when two
projects compete for this, the one defined in last will define the
dependency scope.

Issue: SPR-13013

buildSrc/src/main/groovy/org/springframework/build/gradle/MergePlugin.groovy

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2013 the original author or authors.
+ * Copyright 2002-2015 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -132,8 +132,9 @@ class MergePlugin implements Plugin<Project> {
 						intoConfiguration.dependencies.add(it)
 					}
 				}
+				def index = project.parent.childProjects.findIndexOf {p -> p.getValue() == project}
 				project.merge.into.install.repositories.mavenInstaller.pom.scopeMappings.addMapping(
-					mapping.priority + 100, intoConfiguration, mapping.scope)
+					mapping.priority + 100 + index, intoConfiguration, mapping.scope)
 			}
 		}
 	}