`div` treats plain numbers and unitless quantities differently

This package usually treats plain numbers like `3` and quantities without units (i.e., with the `NoUnits` unit) the same. But `div` doesn’t when the other argument is a dimensionful quantity:
```julia
julia> x = 3
3

julia> y = Quantity{Int, NoDims, typeof(NoUnits)}(3)
3

julia> div(10u"m", x)
3 m

julia> div(10u"m", y)
ERROR: DimensionError: and m are not dimensionally compatible.
Stacktrace:
[...]
```

Start by reproducing the Julia REPL examples and tracing the `div` dispatch involving `Quantity`, `NoUnits`, and dimensionful quantities. Compare the paths for a plain number and a unitless quantity, then verify that both cases behave consistently when dividing `10u"m"`.

Written by the indexing model from the issue text.