Throw against having the same dependency resolved in BDL and as a property

[frenzibyte]

Avoids cases like ppy/osu#25191 (comment) from happening (also allowing for the developer to be hinted at the correct nullability of a dependency).

[bdach]

I disagree with throwing. Should be an analyser at best. (#4571 (comment))

[frenzibyte]

Fair enough.