title: CMake: Check Python3 and Python packages presence
body: <p>I'm currently creating a CMake project in which a couple of Python scripts are called as commands/targets whenever some classic targets are properly executed (classic compilation).</p>
  <p>I want CMake to Check the presence of 2 kind of elements (error if not found):</p>
  <ol>
  <li>Presence of Python interpreter (let's say Python 3 but both Python 2.7 and 3 are compatible),</li>
  <li><strong>Presence of Python packages installed by pip(3)</strong>.</li>
  </ol>
  <p>For the first part, I use that code snippet which seems to work well:</p>
  <pre><code>find_package(
          Python3
          REQUIRED
          COMPONENTS Interpreter
  )
  </code></pre>
  <p><strong>But how to specify that I want some pip packages (like <em>pycryptodome</em>) to be installed?</strong></p>
  <p>Note: The Python scripts I use are third-party scripts which are only run as post-compilation tools: They are not required as development dependency for any target.</p>