Refactor detection for closed plugins

[Zodiac1978]

It looks like, that the closed plugin status is now available via API:
https://meta.trac.wordpress.org/ticket/7057#comment:12

Maybe this is more reliable for the future. We could also wait for the SVN check to break until this is refactored.

So just a heads up, that there seems to be better way now.

[Zodiac1978]

Related: https://github.com/WordPress/wporg-experiments-plugin