Doc: Drop the use of QT_PLUGIN_PATH in documentation config

This caused build issues in certain configurations. We don't actually
need it, as we can rely on the base Qt include path instead.

Also move the custom module header under src/webengine/doc and rename
it.

Task-number: QTBUG-67342
Change-Id: Id5e809202ddaa285bfcc341a36b0b3d5a7ade436
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/webengine/QtWebEngine_pch.h

-/****************************************************************************
-**
-** Copyright (C) 2018 The Qt Company Ltd.
-** Contact: https://www.qt.io/licensing/
-**
-** This file is part of the QtWebEngine module of the Qt Toolkit.
-**
-** $QT_BEGIN_LICENSE:LGPL$
-** Commercial License Usage
-** Licensees holding valid commercial Qt licenses may use this file in
-** accordance with the commercial license agreement provided with the
-** Software or, alternatively, in accordance with the terms contained in
-** a written agreement between you and The Qt Company. For licensing terms
-** and conditions see https://www.qt.io/terms-conditions. For further
-** information use the contact form at https://www.qt.io/contact-us.
-**
-** GNU Lesser General Public License Usage
-** Alternatively, this file may be used under the terms of the GNU Lesser
-** General Public License version 3 as published by the Free Software
-** Foundation and appearing in the file LICENSE.LGPL3 included in the
-** packaging of this file. Please review the following information to
-** ensure the GNU Lesser General Public License version 3 requirements
-** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
-**
-** GNU General Public License Usage
-** Alternatively, this file may be used under the terms of the GNU
-** General Public License version 2.0 or (at your option) the GNU General
-** Public license version 3 or any later version approved by the KDE Free
-** Qt Foundation. The licenses are as published by the Free Software
-** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
-** included in the packaging of this file. Please review the following
-** information to ensure the GNU General Public License requirements will
-** be met: https://www.gnu.org/licenses/gpl-2.0.html and
-** https://www.gnu.org/licenses/gpl-3.0.html.
-**
-** $QT_END_LICENSE$
-**
-****************************************************************************/
-
-// Module header file used for documentation builds only.
-#ifdef Q_CLANG_QDOC
-#include <QtWebEngine>
-#include <QtWebEngineWidgets>
-#endif // Q_CLANG_QDOC

src/webengine/doc/QtWebEngineDoc

+#include <QtWebEngine/QtWebEngine>
+#include <QtWebEngineWidgets/QtWebEngineWidgets>

src/webengine/doc/qtwebengine.qdocconf

@@ -39,11 +39,10 @@ manifestmeta.highlighted.names += "QtWebEngine/WebEngine Widgets Simple Browser
 tagfile                 = ../../../doc/qtwebengine/qtwebengine.tags
 
 # Custom module header for clang that pulls in also QtWebEngineWidgets
-moduleheader = QtWebEngine_pch.h
+moduleheader = QtWebEngineDoc
 
 # Additional include paths for clang
-includepaths = -I $QT_PLUGIN_PATH/../include/QtWidgets \
-               -I $QT_PLUGIN_PATH/../include/QtWebEngineWidgets
+includepaths = -I .
 
 depends += qtcore \
            qtgui \