Rewrite mix-in linking for error reporting, fix bugs.

The semantic change in this diff has to do with how
local signatures are treated.  Previously, we threw them
into the bag when mix-in linking, which means that if
you declared a signature, and another package brought
that module into scope, the signature would get "filled".
This is actually never what you want in non-recursive
Backpack, so this commit moves it so that we only put the
signatures in post-facto.  There is still some more sanity
checking we should do but I'm deferring that for a later
commit.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>