PIP cache should cache the installed packages as well

Description:
Currently, setup-python caches only the ~/.cache/pip directory to avoid redownloads. However, it doesn't cache the installed packages. As some package have lengthy installation steps, this leads to delays in builds.

You can see the current behaviour for example in https://github.com/crabhi/setup-python-cache-test/actions/runs/1789016634 (or in attached build.txt) - the pip install output shows "Collecting" and "Installing" instead of "Requirement already satisfied" for all packages.

Justification:
For example installing the ansible package takes well over a minute even if it's already downloaded.

Are you willing to submit a PR?
Yes, I can try.

首先，跟踪 setup-python 当前对 ~/.cache/pip 的缓存行为，以及链接的复现工作流中的 pip install 步骤。比较缓存包路径和已安装包路径，然后验证当恢复的缓存使 pip 报告 "Requirement already satisfied" 而不是重新安装包时的变化。