fix getting active plugins in multisite environment

[stefanullinger]

As discussed in elementor/wp2static#730 WordPress stores network active plugins of a Wordpress multisite environment in the active_sitewide_plugins option of the wp_sitemeta table. This option can be accessed using get_site_option.

We need to merge these plugins with the blog's active plugins in order to get a list of all active plugins for a single blog.

[leonstafford]

Thanks @stefanullinger, sorry, the settings for repo require me to manually approve first pull request, it seems.

Tests failing in this repo before first release, regardless, so I've merged this in and will do same in WP2Static, thanks so much for putting in both!!