When trying to regenerate zproto-marshal.go on a fresh go workspace I
got:

    (neo) (z-dev) (g.env) kirr@deco:~/src/neo/src/lab.nexedi.com/kirr/neo/go/neo/proto$ go generate
    typecheck: ../../zodb/zodb.go:174:2: could not import lab.nexedi.com/kirr/go123/mem (can't find import: "lab.nexedi.com/kirr/go123/mem")
    exit status 1
    proto.go:57: running "sh": exit status 1

Digging around a bit this turned out to be due to that
importer.Default() is using packages in only binary installed form:

    https://github.com/golang/go/issues/19334#issuecomment-283430322
    https://github.com/golang/go/issues/11415

Fix it by using "source" importer which always looks at sources.

(zproto-marshal.go stays unchanged after `go generate` rerun)