I just noticed that Pkg.dir is used extensively in Documenter.jl to build documentation for other packages. This is a good example of something that needs to find the directory of another package (which may not even be imported yet). (Similar usage can be found in Coverage.jl.)
Both Documenter.jl and Coverage.jl are extensively used in Julia packages, and their usage of Pkg.dir will need a non-deprecated replacement in 0.7.