loading: . in LOAD_PATH redundant; always look in Pkg.dir() first.

This closes #2968 since changes to JULIA_PKGDIR are immediately
reflected in Pkg.dir() which is now always considered when requiring
things before looking in LOAD_PATH. Having "." in LOAD_PATH was
redundant since the find_in_path logic *already* looks for files
that exist first. I suspect that all of this logic needs some
regularization because it's pretty lumpy right now, but this does
the trick and closes the issue without breaking things.
Merge branch 'master' of github.com:JuliaLang/julia