Let install.php detect and inject extensions [2]

Let install.php detect and inject extensions [2]

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.

The original patch 53926d9d was broken and got reverted. This is the
proper one.

Bug: T189567
Change-Id: I41820fce12e01224d64ab711e016835ef4d4febc