Make symbols in deprecation warnings more explicit

[rikhuijzer]

The deprecation warning read:

┌ Warning: `MvNormal(d::Int, σ::Real)` is deprecated, use `MvNormal(Diagonal(Fill(σ ^ 2, d)))` instead.

However, Fill is not exported by Distributions and is likely to cause much searching for many readers.

This PR fixes it by using fill instead of Fill. Another solution would be to export Fill in src/Distributions.jl.

cc: @devmotion

[devmotion]

No, I definitely think users should use Fill - otherwise the behaviour is different than before and their code will become allocating.

Also a no on the second point, we agreed to not re-export FillArrays (and other dependencies) but instead document them clearly: #1368

[devmotion]

I wonder if the deprecation message becomes clearer if we use LinearAlgebra.Diagonal and FillArrays.Fill in the @deprecated definition. Ideally users should see

┌ Warning: `MvNormal(d::Int, σ::Real)` is deprecated, use `MvNormal(LinearAlgebra.Diagonal(FillArrays.Fill(σ ^ 2, d)))` instead.

which would match our (ideal) documentation (all external structs and functions are qualified).

[devmotion]

It seems to work locally (don't pay attention to the stupid example):

julia> using LinearAlgebra

julia> g(x) = sum(abs2, x)
g (generic function with 1 method)

julia> Base.@deprecate f(x) g(LinearAlgebra.Diagonal(x))
f (generic function with 1 method)

julia> f(rand(3, 3))
┌ Warning: f(x) is deprecated, use g(LinearAlgebra.Diagonal(x)) instead.
│   caller = top-level scope at REPL[4]:1
└ @ Core REPL[4]:1
0.2616974132900723

[rikhuijzer]

I wonder if the deprecation message becomes clearer if we use LinearAlgebra.Diagonal and FillArrays.Fill in the @deprecated definition. Ideally users should see

┌ Warning: `MvNormal(d::Int, σ::Real)` is deprecated, use `MvNormal(LinearAlgebra.Diagonal(FillArrays.Fill(σ ^ 2, d)))` instead.

which would match our (ideal) documentation (all external structs and functions are qualified).

I love it!

[devmotion]

Thanks, looks good! While you're at it, maybe update the deprecations in mvnormalcanon.jl as well?

[devmotion]

LGTM, thanks! I'll merge when tests pass.

[devmotion]

Test errors are unrelated (recent release of PDMats uncovered a bug in the implementation of Wishart).