Disallow "import mod.submod as m", because the result is ambiguous. Does it

Disallow "import mod.submod as m", because the result is ambiguous. Does it
load mod.submod as m, or mod as m ? Both can be achieved differently, and
unambiguously. Also attempt to document this restriction (editor
appreciated!)

Note that this is an artificial check during compile, because incorporating
this in the grammar is hard, and then adjusting the compiler to do the right
thing with the right nodes is harder.