base/path.jl: shorten _regex suffixes to _re.
dirname is idempotent, normpath appends / to directories [#2000]

When you do (dir,name) = splitdir(path) dir*name is always path
and now when parsing paths that must be directories like "foo/."
dirname is the whole thing. When normalizing paths that must be
directories like "foo/bar/.." the result will always necessarily
be directories – i.e. they end with "/", "." or "..".