Python: Move plugin class definition to .cpp

The plugin classes partially changed there purpose, they are no more
meant to provide internal utility functionality.

Change-Id: I9b9200995eaa95dcd924c94dcedb28e6d5db0be9
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/python/pipsupport.cpp

@@ -221,7 +221,7 @@ QFuture<PipPackageInfo> Pip::info(const PipPackage &package)
 }
 
 Pip::Pip(const Utils::FilePath &python)
-    : QObject(PythonPlugin::instance())
+    : QObject(pluginInstance())
     , m_python(python)
 {}