Configure Checkstyle to require package-info.java files

This commit updates the project's checkstyle configuration to require
the existence of a package-info.java file for all packages within the
src/main directory while excluding the framework-docs module and
certain packages inside the spring-core module. A missing
package-info.java file will result in emitting a warning.

See gh-30069

src/checkstyle/checkstyle-suppressions.xml

@@ -130,4 +130,13 @@
 	<suppress files="sockjs[\\/]transport[\\/]TransportType" checks="JavadocVariable"/>
 	<suppress files="src[\\/]test[\\/]java[\\/]org[\\/]springframework[\\/]web[\\/]reactive[\\/]protobuf[\\/].*" checks=".*"/>
 
+	<!-- Suppress JavadocPackage checks on packages outside of src/main-->
+	<!-- And outside 'framework-docs' module-->
+	<!-- And outside 'spring-core/src/main/java/org/springframework/asm' package-->
+	<!-- And outside 'spring-core/src/main/java/org/springframework/cglib' package-->
+	<!-- And outside 'spring-core/src/main/java/org/springframework/objenesis' package-->
+	<!-- And outside 'spring-core/src/main/java/org/springframework/javapoet' package-->
+	<!-- And outside 'spring-core/src/main/java/org/springframework/lang' package-->
+	<suppress checks="JavadocPackage" files="(^(?!.*src[\\/]main[\\/]).*)|(.*framework-docs.*)|(.*spring-core\/src\/main\/java\/org\/springframework\/asm.*)|(.*spring-core\/src\/main\/java\/org\/springframework\/cglib.*)|(.*spring-core\/src\/main\/java\/org\/springframework\/objenesis.*)|(.*spring-core\/src\/main\/java\/org\/springframework\/javapoet.*)|(.*spring-core\/src\/main\/java\/org\/springframework\/lang.*)"/>
+
 </suppressions>

src/checkstyle/checkstyle.xml

@@ -240,4 +240,9 @@
 		<module name="io.spring.javaformat.checkstyle.check.SpringJavadocCheck"/>
 		<module name="io.spring.javaformat.checkstyle.check.SpringJUnit5Check"/>
  	</module>
+
+	<!--package-info checker -->
+	<module name="JavadocPackage">
+		<property name="severity" value="warning"/>
+	</module>
 </module>