fix duplicate query searches from same global action keywords

[jjw24]

When a plugin has multiple global action keywords, flow loops through the plugin's global (*) and action keywords and fire them off to query. This creates a scenario where a plugin with multiple global action keywords (*), for example 4, running the plugin search 4 times unnecessarily. This also could potentially have a performance cost to it.

This fix excludes adding multiple duplicate global and action keywords even if the plugin has 4 global action keywords, only fire search once.

[taooceros]

why not just make GlobalPlugins a hashset? that can also fix sometimes people trying to add global actionkeyword twice.

[taooceros]

I finish the edit for HashSet because I would like to make some further change based on this branch.

I found that await Task.Yield won't actually run task in threadpool, so maybe we shouldn't use it here.
When I try to replace the Task.Run with await Task.Yield in Program plugin, it add 10-15ms delay for returning result. Maybe we should assume every async plugins are doing right thing so that we can simply remove the await Task.Yield and let it run sync.

[jjw24]

lgtm, did you want to approve?

[taooceros]

Sorry I am reinstalling windows today. Will do it now