cmd/go/internal/modcmd: skip modules with empty version strings

This CL restores behavior before CL 189797 and fixes a misleading
comment. modload.ListModules may return info without a version for the
main module and for modules replaced with local directories.

Fixes #35505

Change-Id: I5b4e68053a680ff897b072fdf6e7aa17b6e1ac34
Reviewed-on: https://go-review.googlesource.com/c/go/+/206538
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/download.go

@@ -89,7 +89,7 @@ func runDownload(cmd *base.Command, args []string) {
 		for _, arg := range args {
 			switch arg {
 			case modload.Target.Path, targetAtLatest, targetAtUpgrade, targetAtPatch:
-				os.Stderr.WriteString("go mod download: skipping argument "+ arg + " that resolves to the main module\n")
+				os.Stderr.WriteString("go mod download: skipping argument " + arg + " that resolves to the main module\n")
 			}
 		}
 	}
@@ -102,9 +102,9 @@ func runDownload(cmd *base.Command, args []string) {
 		if info.Replace != nil {
 			info = info.Replace
 		}
-		if (module.Version{Path: info.Path, Version: info.Version} == modload.Target) {
-			// skipping main module.
-			// go mod download without dependencies is silent.
+		if info.Version == "" && info.Error == nil {
+			// main module or module replaced with file path.
+			// Nothing to download.
 			continue
 		}
 		m := &moduleJSON{

src/cmd/go/testdata/script/mod_download_replace_file.txt

+# This test checks that 'go mod download' produces no output for
+# the main module (when specified implicitly) and for a module replaced
+# with a file path.
+# Verifies golang.org/issue/35505.
+go mod download -json all
+cmp stdout no-output
+
+-- go.mod --
+module example.com/a
+
+require example.com/b v1.0.0
+
+replace example.com/b => ./local/b
+-- local/b/go.mod --
+module example.com/b
+-- no-output --