cmd/vendor: create modules.txt even for unused deps

`go mod vendor`  should create vendor/modules.txt even when the only deps
in go.mod are unused.

Fixes: #36580

Change-Id: I92a746d3f013bc2bdc3d2cec6e14b16f606c2edd
GitHub-Last-Rev: ea39a1c622
GitHub-Pull-Request: golang/go#36920
Reviewed-on: https://go-review.googlesource.com/c/go/+/217135
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/vendor.go

@@ -123,6 +123,11 @@ func runVendor(cmd *base.Command, args []string) {
 		fmt.Fprintf(os.Stderr, "go: no dependencies to vendor\n")
 		return
 	}
+
+	if err := os.MkdirAll(vdir, 0777); err != nil {
+		base.Fatalf("go mod vendor: %v", err)
+	}
+
 	if err := ioutil.WriteFile(filepath.Join(vdir, "modules.txt"), buf.Bytes(), 0666); err != nil {
 		base.Fatalf("go mod vendor: %v", err)
 	}

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

@@ -0,0 +1,17 @@
+# Ensure that we generate a vendor/modules.txt file even when the only
+# requirements in go.mod are unused.  Regression test for
+# golang.org/issue/36580
+
+env GO111MODULE=on
+
+go mod vendor
+cmp go1.14-modules.txt vendor/modules.txt
+
+-- go.mod --
+module example.com/m
+go 1.14
+
+require example.com v1.0.0 // indirect
+-- go1.14-modules.txt --
+# example.com v1.0.0
+## explicit