better handling of mistaken top-level variable
references during the parsing of :=. the base
problem is that when reading
a,b,c,d
the parser makes those refer to existing variables,
which might create a few stub top-level ones
for undefined names, but then if a := is the next
token, we need to undo those stubs.
this was causing problems in multifile packages
in which one file used a := variable named rpc
and the other imported a package named rpc.