clarify @define_diffrule namespaces

[tpapp]

I think that eg

@define_diffrule SpecialFunctions.lgamma(x) = :( digamma($x) )

should be

@define_diffrule SpecialFunctions.lgamma(x) = :( SpecialFunctions.digamma($x) )

since the RHS is not necessarily evaluated in the module of the LHS. Is this correct?

Just please clarify, I would be happy to make a PR.

(cf JuliaStats/StatsFuns.jl#49)

[jrevels]

Yup, good catch! The other rules should have this as well.

[tpapp]

Can you help me figure out why this wasn't caught by the unit tests? I think they should be updated first, so that we get an error, which is then fixed.

[jrevels]

I think it's because the tests do using SpecialFunctions, which brings all of these bindings into the testing namespace. IIUC, we should be able to fix that just by changing it to import SpecialFunctions.

[tpapp]

Indeed, import breaks the tests, I made a PR which fixes things.

A review would be appreciated, especially for the case I mention in the PR discussion.