Missed simplification opportunity for `productmeasure`

[cscherrer]

In this measure

julia> d = productmeasure(fill(Lebesgue(), 5))
ProductMeasure([Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ), Lebesgue(ℝ)])

we have

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.