Thanks for the patch, Nieraj. I have committed it with some changes. I had originally hoped to use your patch largely as-is for type completion proposal ordering, but unfortunately, I had to make some fairly significant changes.
With quickfixes, you have access to the IType that you are proposing, but during content assist, you do not. There is only the fully qualified name, modifiers, and accessibility available. This means that rules like source/binary and same project cannot be calculated during content assist. Also, this means that I had to add a second method to IRelevanceRule that is applicable during content assist.
The result is something that is less clean than your original patch, but it is working in both situations. If you want to clean things up, then feel free to, but I think what we have now is sufficient.