internal/worker: enforce go.mod dependency download for args.init

- Previously, go.mod dependencies were not fully downloaded when
  args.init was true. This change serves to ensure that go.mod
  dependencies are downloaded if the file exists. Otherwise, it will be
  created with `go mod init && go mod tidy`.

Change-Id: Idd631f5a05dd394d8f39bfc1ac766f181e822a83
Reviewed-on: https://go-review.googlesource.com/c/pkgsite-metrics/+/705935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Zvonimir Pavlinovic <zpavlinovic@google.com>
Auto-Submit: Ethan Lee <ethanalee@google.com>

internal/worker/scan.go

diff --git a/internal/worker/scan.go b/internal/worker/scan.go
index babae13..0bc5e8b 100644
--- a/internal/worker/scan.go
+++ b/internal/worker/scan.go
@@ -250,22 +250,32 @@
 // prepareModule prepares a module for scanning, and takes other actions that increase
 // the chance that package loading will succeed.
 func prepareModule(ctx context.Context, args prepareModuleArgs) error {
-	if args.noDeps || args.init {
-		log.Infof(ctx, "downloading %s@%s to %s", args.modulePath, args.version, args.dir)
-		if err := modules.Download(ctx, args.modulePath, args.version, args.dir, args.proxyClient); err != nil {
-			return err
-		}
-		hasGoMod := fileExists(filepath.Join(args.dir, "go.mod"))
-		if !hasGoMod && args.init {
-			return goModInit(ctx, args.modulePath, args.version, args.dir, args.modulePath, args.insecure)
-		}
-		return nil
-	}
-
 	opts := &goCommandOptions{
 		dir:      args.dir,
 		insecure: args.insecure,
 	}
+	if args.noDeps || args.init {
+		log.Infof(ctx, "downloading %s@%s to %s", args.modulePath, args.version, args.dir)
+		if err := modules.Download(ctx, args.modulePath, args.version, args.dir, args.proxyClient); err != nil {
+			log.Warnf(ctx, "`modules.Download` error: %v", err)
+			return err
+		}
+
+		if args.noDeps {
+			log.Infof(ctx, "Skipping downloading modules for %s@%s since args.dir is %t", args.modulePath, args.version, args.noDeps)
+			return nil
+		}
+
+		hasGoMod := fileExists(filepath.Join(args.dir, "go.mod"))
+		if hasGoMod {
+			return runGoCommand(ctx, args.modulePath, args.version, opts, "mod", "download")
+		} else {
+			if err := goModInit(ctx, args.modulePath, args.version, args.dir, args.modulePath, args.insecure); err != nil {
+				return err
+			}
+			return goModTidy(ctx, args.modulePath, args.version, args.dir, args.insecure)
+		}
+	}
 
 	log.Infof(ctx, "prepareModule %s@%s to %s", args.modulePath, args.version, args.dir)
 	if err := runGoCommand(ctx, args.modulePath, args.version, opts, "mod", "init", "an.empty/module"); err != nil {