PluginClassLoader 改进加载 com.alibaba.oneagent package下面的类的逻辑

PluginClassLoader 加载 com.alibaba.oneagent package下面的类，应该优先从 com.alibaba.oneagent.plugin.PluginManager 所在的ClassLoader加载。 避免应用可能在 System classloader里有依赖到 `com.alibaba.oneagent ` 里的类？

Start by locating PluginClassLoader and the com.alibaba.oneagent.plugin.PluginManager class loader entry point. Trace how classes under com.alibaba.oneagent are selected, then verify that this package is loaded with PluginManager's ClassLoader before any System classloader dependency. The issue provides no test or file path, so completion should be confirmed with a focused loading scenario.

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