Missed simplification opportunity for `productmeasure`

In this measure
```julia
julia> d = productmeasure(fill(Lebesgue(), 5))
ProductMeasure([Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ)])
```

we have
```julia
julia> Base.issingletontype(eltype(marginals(d)))
true
```

But this guarantees we could instead have written it as a power measure. Let's update the `powermeasure` to account for this.