Support inferred base package for @ComponentScan

Prior to this change, @ComponentScan required the declaration of
exactly one of the #value, #basePackage or #basePackageClasses
attributes in order to determine which package(s) to scan.

This commit introduces support for base package inference, relaxing the
above requirement and falling back to scanning the package in which the
@ComponentScan-annotated class is declared.

Issue: SPR-9586

spring-context/src/test/java/example/scannable_implicitbasepackage/ScannedComponent.java

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2002-2012 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package example.scannable_implicitbasepackage;
+
+import org.springframework.stereotype.Component;
+
+/**
+ * @author Phillip Webb
+ */
+@Component
+public class ScannedComponent {
+
+}