go/neo/proto: Fix protogen to work without prior `go install`

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)

go/neo/proto/protogen.go

-// Copyright (C) 2016-2018  Nexedi SA and Contributors.
+// Copyright (C) 2016-2020  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -145,7 +145,11 @@ func (li *localImporter) Import(path string) (*types.Package, error) {
 
 // importer instance - only 1 so that for 2 top-level packages same dependent
 // packages are not reimported several times.
-var localImporterObj = &localImporter{importer.Default()}
+//
+// don't use importer.Default - this importer uses only binaries for installed
+// packages, which might a) get stale wrt sources, or b) be completely missing.
+// https://github.com/golang/go/issues/11415
+var localImporterObj = &localImporter{importer.For("source", nil)}
 
 func loadPkg(pkgPath string, sources ...string) *types.Package {
 	var filev []*ast.File