Close #300, thanks kviktor

In https://github.com/probonopd/linuxdeployqt/issues/300#issuecomment-398049462 @kviktor writes: 

> With this workaround things seem to work great (originally I just removed the whole if/return part but with that the program ran for about 15 mins, with the current one it takes around 5 min to complete which is close to the original one, also it fixes the issue that `xcbglintegrations` is not getting copied over)

tools/linuxdeployqt/shared.cpp

@@ -1504,8 +1504,15 @@ void deployQmlImport(const QString &appDirPath, const QSet<QString> &rpaths, con
 
     // Skip already deployed imports. This can happen in cases like "QtQuick.Controls.Styles",
     // where deploying QtQuick.Controls will also deploy the "Styles" sub-import.
-    if (QDir().exists(importDestinationPath))
+    // NOTE: this stops the deployment of certain imports if there is a folder in them, for example
-        return;
+    //       - if QtQuick.Controls.Styles is already there QtQuick.Controls is skipped
+    //       - if QtQuick.Extras.Private is there QtQuick.Extras is skipped
+    QDir destDir(importDestinationPath);
+    if (destDir.exists()) {
+        destDir.setFilter(QDir::Files | QDir::NoDotAndDotDot);
+        if(destDir.entryInfoList().length() > 0)
+            return;
+    }
 
     recursiveCopyAndDeploy(appDirPath, rpaths, importSourcePath, importDestinationPath);
 }