Install from @latest, retract v0.0.1, print a go-installed binary's version (closes #116)

README "Install" now leads with go install ...@latest: until 1.0 each
release is a v0.0.N tag on main's tip, so @latest gives that tip once
it has passed the build. It also says what @main and v0.0.1 give and
how to check an installed binary. go.mod retracts v0.0.1, the early
build without a database.

sfdupes --version printed "dev" for every go-installed binary, since
only the linker set Version. When the linker sets none, it now prints
the main module's version from runtime/debug.ReadBuildInfo; "dev" is
left only when neither is known. Version is now empty by default.

The retraction takes effect only once a release carrying it is the
newest.

Model: opus-5-5

main.go

@@ -28,6 +28,7 @@ import (
 	"io"
 	"os"
 	"runtime"
+	"runtime/debug"
 
 	"github.com/spf13/cobra"
 )
@@ -58,10 +59,32 @@ var errNoSubcommand = errors.New("no subcommand")
 var errWorkersBelowOne = errors.New("--workers must be at least 1")
 
 // Version is the build version, injected at link time via -ldflags
-// (see the Makefile); "dev" for a plain go build.
+// (see the Makefile); empty when the linker set none, as for a binary
+// built by go install. buildVersion turns it into what --version prints.
 //
 //nolint:gochecknoglobals // written only by the linker
-var Version = "dev"
+var Version string
+
+// buildVersion returns the version that sfdupes --version prints
+// (README "Install"): the version the linker set, when it set one;
+// otherwise the main module's version that Go recorded in the binary,
+// a tag such as v0.0.2 or the pseudo-version of an @main build; and
+// "dev" when neither is known. info is what debug.ReadBuildInfo
+// returns, nil when the binary carries none. Go records "(devel)" for
+// a build that has no version, such as one inside a copy of the source
+// without git.
+func buildVersion(linkerVersion string, info *debug.BuildInfo) string {
+	if linkerVersion != "" {
+		return linkerVersion
+	}
+
+	if info == nil || info.Main.Version == "" ||
+		info.Main.Version == "(devel)" {
+		return "dev"
+	}
+
+	return info.Main.Version
+}
 
 func main() {
 	// Once the reader of a stdout pipe has gone, as in "sfdupes report |
@@ -116,7 +139,10 @@ func newRootCommand(stdout, stderr io.Writer) *cobra.Command {
 	var showVersion bool
 
 	printVersion := runE(func(context.Context, []string) error {
-		_, err := fmt.Fprintf(stdout, "sfdupes %s\n", Version)
+		info, _ := debug.ReadBuildInfo()
+
+		_, err := fmt.Fprintf(stdout, "sfdupes %s\n",
+			buildVersion(Version, info))
 		if err != nil {
 			return fmt.Errorf("write stdout: %w", err)
 		}