`pkgdir` returns wrong dir for extension if extension is in its own folder

[IanButterworth]

julia/base/loading.jl

Line 528 in 0c0fdf0

return joinpath(dirname(dirname(path)), paths...)

Assumes the result of pathof is a .../src/Foo.jl path. Extensions can be located at .../ext/FooExt.jl or .../ext/FooExt/FooExt.jl