[6.x] Add explicit package param to installed tag

[daun]

Allow using the installed tag with a param. Currently, only wildcard usage is supported.

{{ package_name = "scope/package" }}

{{ installed :package="package_name" }}yes{{ /installed }}

[jasonvarga]

index() passes null into Composer::isInstalled(string $package) when package is missing/empty, throwing a TypeError and crashing the render (previously {{ installed }} just returned empty). Easy to hit via {{ installed :package="var" }} when var is unset. Can you guard it?

public function index()
{
    if (! $package = $this->params->get('package')) {
        return;
    }

    return $this->installed($package);
}

[daun]

@jasonvarga Right, good catch! I've pushed your proposed change.