Support replacing submodule's public url (qt-) with commercial url (tqtc-)

We need to support tqtc-qt5 builds with some submodule(s) replaced to tqtc specific one.

At the moment we can do this partially, see https://codereview.qt-project.org/c/qt/tqtc-qt5/+/283013 . But the problem there is that we have also change submodule name in .gitmodule and so on it is also affecting generated src packages etc, see http://ci-files02-hki.intra.qt.io/packages/jenkins/qt/vxworks-5.9/latest/src/submodules/

So we should allow us to change only repository and branch but keep the name as it is - meaning we should be able to define:

[submodule "qtbase"]
path = qtbase
url = ../tqtc-qtbase.git
branch = tqtc/vxworks-5.9
status = essential

At the moment this functionality is needed to get vxworks packages from coin but in the (near) future there will be some other, similar kind of new products