DATAJPA-834 - Use less common bean name for EntityManagerBeanDefinitionRegistrarPostProcessor.

Replacing bean name "foo" with more meaningful value injected via constant as a bean probably shouldn't have "foo" as its name when everything else has meaningful values defined in constants. Renamed bean and extracted constant alongside others referenced within the file.

Original pull request: #159.

src/main/java/org/springframework/data/jpa/repository/config/BeanDefinitionNames.java

@@ -24,4 +24,5 @@ package org.springframework.data.jpa.repository.config;
 interface BeanDefinitionNames {
 
 	public static final String JPA_MAPPING_CONTEXT_BEAN_NAME = "jpaMappingContext";
+	public static final String EM_BEAN_DEFINITION_REGISTRAR_POST_PROCESSOR_BEAN_NAME = "emBeanDefinitionRegistrarPostProcessor";
 }

src/main/java/org/springframework/data/jpa/repository/config/JpaRepositoryConfigExtension.java

@@ -163,7 +163,7 @@ public class JpaRepositoryConfigExtension extends RepositoryConfigurationExtensi
 		Object source = config.getSource();
 
 		registerIfNotAlreadyRegistered(new RootBeanDefinition(EntityManagerBeanDefinitionRegistrarPostProcessor.class),
-				registry, "foo", source);
+				registry, EM_BEAN_DEFINITION_REGISTRAR_POST_PROCESSOR_BEAN_NAME, source);
 
 		registerIfNotAlreadyRegistered(new RootBeanDefinition(JpaMetamodelMappingContextFactoryBean.class), registry,
 				JPA_MAPPING_CONTEXT_BEAN_NAME, source);