plugin: Check source package's name also for multiple whitelist entries

This patch is prework for supporting unit test of other plugins on Windows.

Unlike *nix build which create registry per test case, we don't create that for Windows, and eventually plugin scanner will check whether a plugin belongs to whitelist or not.

As an example of failure, coreelement's corresponding GstPluginDesc stores "source" as "gstreamer" and "name" as "coreelement". Then, if a unit test set whitelist to "gstreamer,gst-plugins-base", the "name" (coreelement in this case) never be matched with "gstreamer" and finally the coreelement plugin couldn't be loaded.