cmd/mkpkg: support specifying that a package replaces another.

Both RPM and Deb require us to specify both Replaces and Conflicts:
Conflicts tells them that the packages cannot coexist on the system,
Replaces tells them which one to keep.

cmd/mkpkg/main.go

@@ -41,6 +41,7 @@ func main() {
 	postinst := getopt.StringLong("postinst", 0, "", "debian postinst script path")
 	prerm := getopt.StringLong("prerm", 0, "", "debian prerm script path")
 	postrm := getopt.StringLong("postrm", 0, "", "debian postrm script path")
+	replaces := getopt.StringLong("replaces", 0, "", "package which this package replaces, if any")
 	getopt.Parse()
 
 	filesMap, err := parseFiles(*files)
@@ -66,6 +67,11 @@ func main() {
 		},
 	})
 
+	if *replaces != "" {
+		info.Overridables.Replaces = []string{*replaces}
+		info.Overridables.Conflicts = []string{*replaces}
+	}
+
 	switch *pkgType {
 	case "deb":
 		info.Section = "net"