make import/using .Foo do a relative search for Foo, and the old syntax search from Main (the top level).
addresses #2375

this is quite disruptive, but I think I prefer it. after all, most of
the imports in Base are of internal modules, so they should be marked as
such.
Merge branch 'master' of github.com:JuliaLang/julia into jb/relativeimport
make relative imports explicit based on the number of leading dots
Merge branch 'master' of github.com:JuliaLang/julia into jb/relativeimport