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.

まず、リンク先の再現ワークフローにおける setup-python の現在の ~/.cache/pip のキャッシュ動作と pip install ステップを追跡します。キャッシュされたパッケージとインストールされたパッケージのパスを比較し、復元されたキャッシュによって pip がパッケージを再インストールせずに "Requirement already satisfied" と報告する場合の変更を確認します。