internal/scan: exit after printing version info

When -version flag is passed, print the version information and exit
immediately without trying to do any scanning. This is different than
the behavior of the -show flag, which prints information and then
continues with the scan.

This prevents govulncheck from returning a non-zero exit code when
-version is passed, without any package patterns.

Fixes golang/go#78743

Change-Id: I9b1efa56f067e209956bd38cd77d7055fa70a7ce
Reviewed-on: https://go-review.googlesource.com/c/vuln/+/767300
Reviewed-by: Ethan Lee <ethanalee@google.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/scan/flags.go

diff --git a/internal/scan/flags.go b/internal/scan/flags.go
index 5512540..e67c0a1 100644
--- a/internal/scan/flags.go
+++ b/internal/scan/flags.go
@@ -25,6 +25,7 @@
 	test     bool
 	show     ShowFlag
 	format   FormatFlag
+	version  bool
 	env      []string
 }
 
@@ -72,6 +73,7 @@
 	cfg.patterns = flags.Args()
 	if version {
 		cfg.show = append(cfg.show, "version")
+		cfg.version = true
 	}
 	cfg.ScanLevel = govulncheck.ScanLevel(scanFlag)
 	cfg.ScanMode = govulncheck.ScanMode(modeFlag)

internal/scan/run.go

diff --git a/internal/scan/run.go b/internal/scan/run.go
index f29b9d3..5f6a641 100644
--- a/internal/scan/run.go
+++ b/internal/scan/run.go
@@ -55,6 +55,12 @@
 		return err
 	}
 
+	if cfg.version {
+		// If the -version flag is passed, exit before doing anything else. This is different than
+		// passing -show which includes "version".
+		return nil
+	}
+
 	incTelemetryFlagCounters(cfg)
 
 	switch cfg.ScanMode {