Repackage Javapoet in org.springframework.javapoet

This commit repackages the Javapoet library into spring-core so that it
can be used by the AOT engine without requiring a specific version.

Closes gh-27828

spring-core/src/main/java/org/springframework/javapoet/package-info.java

@@ -0,0 +1,6 @@
+/**
+ * Spring's repackaging of
+ * <a href="https://github.com/square/javapoet">JavaPoet</a>
+ * (with Spring-specific utilities; for internal use only).
+ */
+package org.springframework.javapoet;