switch to use go module support

versions of all dependencies remain the same, but now use `go mod` to
manage them rather than govendor.  This does result in a few extra files
being checked in, since govendor would ignore non-build files and go mod
does not.

vendor/github.com/rwcarlsen/goexif/exif/regen_regress.go

@@ -0,0 +1,79 @@
+// +build ignore
+
+package main
+
+import (
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"os"
+	"path/filepath"
+	"strings"
+
+	"github.com/rwcarlsen/goexif/exif"
+	"github.com/rwcarlsen/goexif/tiff"
+)
+
+func main() {
+	flag.Parse()
+	fname := flag.Arg(0)
+
+	dst, err := os.Create(fname)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer dst.Close()
+
+	dir, err := os.Open("samples")
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer dir.Close()
+
+	names, err := dir.Readdirnames(0)
+	if err != nil {
+		log.Fatal(err)
+	}
+	for i, name := range names {
+		names[i] = filepath.Join("samples", name)
+	}
+	makeExpected(names, dst)
+}
+
+func makeExpected(files []string, w io.Writer) {
+	fmt.Fprintf(w, "package exif\n\n")
+	fmt.Fprintf(w, "var regressExpected = map[string]map[FieldName]string{\n")
+
+	for _, name := range files {
+		f, err := os.Open(name)
+		if err != nil {
+			continue
+		}
+
+		x, err := exif.Decode(f)
+		if err != nil {
+			f.Close()
+			continue
+		}
+
+		fmt.Fprintf(w, "\"%v\": map[FieldName]string{\n", filepath.Base(name))
+		x.Walk(&regresswalk{w})
+		fmt.Fprintf(w, "},\n")
+		f.Close()
+	}
+	fmt.Fprintf(w, "}")
+}
+
+type regresswalk struct {
+	wr io.Writer
+}
+
+func (w *regresswalk) Walk(name exif.FieldName, tag *tiff.Tag) error {
+	if strings.HasPrefix(string(name), exif.UnknownPrefix) {
+		fmt.Fprintf(w.wr, "\"%v\": `%v`,\n", name, tag.String())
+	} else {
+		fmt.Fprintf(w.wr, "%v: `%v`,\n", name, tag.String())
+	}
+	return nil
+}