internal/version: make version available when installed via go install github.com/elastic/elastic-package (#648)

This makes to output of elastic-package version informative when the go.mod-based
install within integrations is used.

Author: efd6

internal/version/version.go

@@ -5,6 +5,7 @@
 package version
 
 import (
+	"runtime/debug"
 	"strconv"
 	"time"
 )
@@ -20,6 +21,17 @@ var (
 	Tag string
 )
 
+// Set Tag to version stored in modinfo if it is not available from the builder.
+func init() {
+	if Tag != "" {
+		return
+	}
+	info, ok := debug.ReadBuildInfo()
+	if ok && info.Main.Version != "(devel)" {
+		Tag = info.Main.Version
+	}
+}
+
 // BuildTimeFormatted method returns the build time preserving the RFC3339 format.
 func BuildTimeFormatted() string {
 	if BuildTime == "unknown" {