Ensure all packages declare package-info.java with null-safety annotations

This commit picks up where the two previous commits left off.

Specifically, this commit:

- Removes the "severity=warning" configuration to ensure that violations
  actually fail the build.
- Fixes regular expressions for suppressions by matching forward
  slashes using `[\\/]` instead of `\/`.
- Moves the configuration for newly introduced checks to locations in
  checkstyle.xml that align with the existing organization of that file.
- Renames the IDs for RegexpSinglelineJava checks from
  javaDocPackageNonNullApiAnnotation/javaDocPackageNonNullFieldsAnnotation
  to packageLevelNonNullApiAnnotation/packageLevelNonNullFieldsAnnotation,
  respectively, since these checks are not related to Javadoc.
- Simplifies the null-safety annotation checks to match against
  imported annotation types, which enforces consistency across
  package-info.java files for the annotation declarations.
- Simplifies the RegEx for JavadocPackage suppressions to only exclude
  packages not under src/main/java (vs src/main) and those in the
  framework-docs module.
- Consistently suppresses all checks for the `asm`, `cglib`, `objenesis`,
  and `javapoet` packages in spring-core.
- Adds explicit suppressions for null-safety annotations for the `lang`
  package in spring-core.
- Adds explicit suppressions for null-safety annotations for the
  `org.aopalliance` package in spring-aop.
- Revises the RegEx for null-safety annotation suppressions to only
  exclude package-info.java files not under src/main/java and
  additionally to exclude package-info.java files in the framework-docs
  module as well as those in the spring-context-indexer,
  spring-instrument, and spring-jcl modules.
- Adds all missing package-info.java files.
- Adds null-safety annotations to package-info.java files where
  appropriate.

Closes gh-30069

spring-test/src/main/java/org/springframework/test/context/event/annotation/package-info.java

@@ -1,4 +1,9 @@
 /**
  * Test execution event annotations for the <em>Spring TestContext Framework</em>.
  */
+@NonNullApi
+@NonNullFields
 package org.springframework.test.context.event.annotation;
+
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-test/src/main/java/org/springframework/test/context/event/package-info.java

@@ -1,4 +1,9 @@
 /**
  * Test event support classes for the <em>Spring TestContext Framework</em>.
  */
+@NonNullApi
+@NonNullFields
 package org.springframework.test.context.event;
+
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-test/src/main/java/org/springframework/test/package-info.java

@@ -1,6 +0,0 @@
-/**
- * Generic unit testing support classes.
- *
- * @see org.springframework.test.util
- */
-package org.springframework.test;

spring-test/src/main/java/org/springframework/test/web/package-info.java

@@ -1,4 +1,9 @@
 /**
  * Helper classes for unit tests based on Spring's web support.
  */
+@NonNullApi
+@NonNullFields
 package org.springframework.test.web;
+
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;