[dev.link] cmd/link: add defined package symbols first, before any non-package symbols
Currently, the loader adds defined package symbols and
non-package symbols to the global index space object by object.
This CL changes it to add all the defined package symbols first,
then all the non-package symbols. The advantage of doing this is
that when adding package symbols, by definition they cannot be
dup to each other, so we don't need to do a name lookup when
adding them.
We still add them to the lookup table (for now), since they may
still be referenced by name (e.g. through linkname). This CL is
also a prerequisite if we want to move to not adding package
symbols to the lookup table entirely (e.g. by using
pre-generated in-file lookup table).
Also update some comments to reflect the current state.