@TestOnly should be allowed for ElementType.PACKAGE

I have a maven module which should only be used for tests in other modules.

Therefore, it would be very useful to be able to provide the package with `@TestOnly` in a `package-info.java` and not to provide every single class of the module with it.

首先阅读 @TestOnly 声明及其 ElementType 目标。检查 package-info.java 如何应用该注解，然后验证包级别的使用已被接受，并且现有的项目检查通过。

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