Remove Package/Tests folders from namespace providers

According to the unity package layout [here](https://docs.unity3d.com/Manual/cus-layout.html), the Tests folder should be in the package root, but they aren't removed from the namespace providers like the Runtime folder, leading to have to manually exclude them for every package. Since they have asmdefs they should probably not be providers.

Start by locating the namespace-provider handling for Unity package folders and compare how the Runtime folder is excluded. Check how asmdefs affect provider selection; done when Package and Tests folders are no longer exposed as namespace providers and no longer require manual exclusion.

Written by the indexing model from the issue text.