[bridges] unify IndexInVector

[odow]

I don't think we need to do this. I tried it out and it actually makes things slightly less readable, because the Variable and Constraint modules need to have MOIB.IndexInVector. It's a trivial type that isn't the source of our problems, and the difference isn't even really measurable.

[blegat]

From withing the module, you need MOIB.IndexInVector instead of IndexInVector but from outside MOI, it replaces MOIB.Constraint.IndexInVector so it improves readability.
Some user might be confused using the wrong type and getting errors. Merging them should avoid this confusion.

[odow]

Okay. Done.