public Module::getInstallDirectory()
Returns the directory where a module should be installed.
If the module is already installed, drupal_get_path() will return a valid path and we should install it there. If we're installing a new module, we always want it to go into /modules, since that's where all the documentation recommends users install their modules, and there's no way that can conflict on a multi-site installation, since the Update manager won't let you install a new module if it's alre