Refine inner class detection algorithm

Update the ConfigurationMetadataAnnotationProcessor nested class
algorithm to prevent inner classes being added as both groups and
properties.

Fixes gh-1975

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataAnnotationProcessorTests.java

@@ -236,17 +236,14 @@ public class ConfigurationMetadataAnnotationProcessorTests {
 		assertThat(metadata, containsProperty("config.first.name"));
 		assertThat(metadata, containsProperty("config.first.bar.name"));
 		assertThat(metadata,
-				containsProperty("config.the-second", InnerClassProperties.Foo.class)
+				containsGroup("config.the-second", InnerClassProperties.Foo.class)
 						.fromSource(InnerClassProperties.class));
 		assertThat(metadata, containsProperty("config.the-second.name"));
 		assertThat(metadata, containsProperty("config.the-second.bar.name"));
 		assertThat(metadata, containsGroup("config.third").ofType(SimplePojo.class)
 				.fromSource(InnerClassProperties.class));
 		assertThat(metadata, containsProperty("config.third.value"));
-		assertThat(
-				metadata,
-				containsProperty("config.fourth").ofType(
-						InnerClassProperties.Fourth.class));
+		assertThat(metadata, containsProperty("config.fourth"));
 		assertThat(metadata, not(containsGroup("config.fourth")));
 	}
 

spring-boot-tools/spring-boot-configuration-processor/src/test/java/org/springframework/boot/configurationprocessor/ConfigurationMetadataMatchers.java

@@ -119,10 +119,13 @@ public class ConfigurationMetadataMatchers {
 			ConfigurationMetadata metadata = (ConfigurationMetadata) item;
 			ItemMetadata property = getFirstPropertyWithName(metadata, this.name);
 			if (property == null) {
-				description.appendText("missing property " + this.name);
+				description.appendText("missing "
+						+ this.itemType.toString().toLowerCase() + " " + this.name);
 			}
 			else {
-				description.appendText("was property ").appendValue(property);
+				description.appendText(
+						"was " + this.itemType.toString().toLowerCase() + " ")
+						.appendValue(property);
 			}
 		}