Fix MergePlugin transitive dependencies

A little terminiology first:

* merge.from - a project that contains source that will be merged
               into merge.into
* merge.into - a project that contains source code that will have
               code from merge.from merged into it.

Previously a module that dependended on merge.into would not see
the merge.from module as a transitive dependency. This worked fine
from a Gradle build because all the code from merge.from is merged
into the merge.into jar. However, in an IDE it did not work because
the IDE does not assemble a jar.

This fix ensures that merge.from modules are automatically added
to the classpath of any module relying on the merge.into project.

Fixes SPR-14650

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

@@ -64,6 +64,7 @@ class MergePlugin implements Plugin<Project> {
 		project.plugins.apply(IdeaPlugin)
 
 		MergeModel model = project.extensions.create("merge", MergeModel)
+		model.project = project
 		project.configurations.create("merging")
 		Configuration runtimeMerge = project.configurations.create("runtimeMerge")
 
@@ -76,6 +77,7 @@ class MergePlugin implements Plugin<Project> {
 			if (it.merge.into != null) {
 				setup(it)
 			}
+			setupIdeDependencies(it)
 		}
 
 		// Hook to build runtimeMerge dependencies
@@ -114,6 +116,16 @@ class MergePlugin implements Plugin<Project> {
 		}
 	}
 
+	private void setupIdeDependencies(Project project) {
+		project.configurations.each { c ->
+			c.dependencies.findAll( { it instanceof org.gradle.api.artifacts.ProjectDependency } ).each { d ->
+				d.dependencyProject.merge.from.each { from ->
+					project.dependencies.add("runtimeMerge", from)
+				}
+			}
+		}
+	}
+
 	private void setupMaven(Project project) {
 		project.configurations.each { configuration ->
 			Conf2ScopeMapping mapping = project.conf2ScopeMappings.getMapping([configuration])
@@ -154,5 +166,12 @@ class MergePlugin implements Plugin<Project> {
 }
 
 class MergeModel {
+	Project project;
 	Project into;
+	List<Project> from = [];
+
+	public void setInto(Project into) {
+		this.into = into;
+		into.merge.from.add(project);
+	}
 }