Possible NRE fix

[IgorFedchenko]

Close #90

Not sure if this is related, but due to ConcurrentDictionary.AddOrUpdate specific internal hashset could be populated with some threading issues.

Other way could be to just throw away Parallel.ForEach here, but... Lets try this update first.

P.S. NRE is caused by the fact that some of the elements of this hashset is null. Each element is taken directly from projectFile, which not be null by itself (it is created just before passing to FindProjectImports method, and used several times before adding it to hashset).
So, the only weak place is adding to hashset, and since updateValueFactory is executed by ConcurrentDictionary without thread lock, this may give some issues. Do not see relation with NRE, but this is only place I can fix here.

[Aaronontheweb]

That makes sense - an immutability problem. Well, using an immutable value for this if we're modifying its contents concurrently makes sense. Let's give that a try.