Let install.php detect and inject extensions

Let install.php detect and inject extensions

For CI, I went with a dirty trick to find extensions and either
wfLoadExtension() or include them. That has since been ported to
Installer::findExtensions() and LocalSettingsGenerator::getText().

The WebInstaller() relies on that to detect extensions and let the user
tick the ones to be installed.

Add --with-extensions to install.php so one can include all extensions
from the command line.

Note: The CliInstaller always wfLoad() any skin it can find.

Bug: T189567
Change-Id: Ic8aa6bea4e7294e54fd5b71df267c9e934f78fce