cmd/go/internal/list: make a copy of the package before zeroing fields
go list -json=<fields> zeroes out the fields in the package struct
that aren't specified. The problem with this is that some of the fields
have references into other fields: specifically, the NoGoError's
Error() function accesses the package struct's Dir field, so if we
clear it out the error will just print out "no Go files in" without a
directory. Instead, make a copy of the package struct before we zero
out the fields so the original values are still there.