I’ve now created a package (https://github.com/yha/FindDefinition.jl) for solving this problem, using hints from the above issue:
julia> using StatsPlots
julia> using FindDefinition
julia> methods(StatsPlots.qqplot)
# 1 method for generic function "qqplot":
[1] qqplot(args...; kw...) in StatsPlots at [...]\.julia\packages\RecipesBase\92zOw\src\RecipesBase.jl:357
julia> finddefs(StatsPlots.qqplot)
1-element Array{LineNumberNode,1}:
:(#= [...]\.julia\packages\StatsPlots\IMY3F\src\distributions.jl:96 =#)