command/fmt: Ensure all variable files ending in .pkrvars.hcl get formatted (#10377)

Before change
```
⇶  packer fmt -check /tmp/unformatted.pkrvars.hcl
Error: Cannot tell whether /tmp/unformatted.pkrvars.hcl contains HCL2 configuration data

⇶  echo $?
1
```

After fix
```
⇶  packer fmt -check /tmp/unformatted.pkrvars.hcl
/tmp/unformatted.pkrvars.hcl

⇶  echo $?
3

⇶  packer fmt -check command/test-fixtures/fmt
command/test-fixtures/fmt/unformatted.pkr.hcl
command/test-fixtures/fmt/unformatted.auto.pkrvars.hcl
command/test-fixtures/fmt/unformatted.pkrvars.hcl

```

command/fmt.go

@@ -72,7 +72,7 @@ func (*FormatCommand) Help() string {
 Usage: packer fmt [options] [TEMPLATE]
 
   Rewrites all Packer configuration files to a canonical format. Both
-  configuration files (.pkr.hcl) and variable files (.pkrvars) are updated.
+  configuration files (.pkr.hcl) and variable files (.pkrvars.hcl) are updated.
   JSON files (.json) are not modified.
 
   If TEMPATE is "." the current directory will be used. The given content must

command/fmt_test.go

@@ -0,0 +1,52 @@
+package command
+
+import (
+	"path/filepath"
+	"strings"
+	"testing"
+
+	packersdk "github.com/hashicorp/packer/packer-plugin-sdk/packer"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestFmt(t *testing.T) {
+	s := &strings.Builder{}
+	ui := &packersdk.BasicUi{
+		Writer: s,
+	}
+	c := &FormatCommand{
+		Meta: testMeta(t),
+	}
+
+	c.Ui = ui
+
+	args := []string{"-check=true", filepath.Join(testFixture("fmt"), "formatted.pkr.hcl")}
+	if code := c.Run(args); code != 0 {
+		fatalCommand(t, c.Meta)
+	}
+	expected := ""
+	assert.Equal(t, expected, strings.TrimSpace(s.String()))
+}
+
+func TestFmt_unformattedPKRVarsTemplate(t *testing.T) {
+	c := &FormatCommand{
+		Meta: testMeta(t),
+	}
+
+	args := []string{"-check=true", filepath.Join(testFixture("fmt"), "unformatted.pkrvars.hcl")}
+	if code := c.Run(args); code != 3 {
+		fatalCommand(t, c.Meta)
+	}
+}
+
+func TestFmt_unfomattedTemlateDirectory(t *testing.T) {
+	c := &FormatCommand{
+		Meta: testMeta(t),
+	}
+
+	args := []string{"-check=true", filepath.Join(testFixture("fmt"), "")}
+
+	if code := c.Run(args); code != 3 {
+		fatalCommand(t, c.Meta)
+	}
+}

command/test-fixtures/fmt/formatted.pkr.hcl

@@ -0,0 +1,7 @@
+source "null" "example" {
+  communicator = "none"
+}
+
+build {
+  sources = ["source.null.example"]
+}

command/test-fixtures/fmt/unformatted.auto.pkrvars.hcl

@@ -0,0 +1 @@
+region ="us-west-2"

command/test-fixtures/fmt/unformatted.pkr.hcl

@@ -0,0 +1,11 @@
+variable "region" {
+  type =string
+}
+
+source "amazon-ebs" "example" {
+  region = var.region
+}
+
+build {
+  sources = ["source.amazon-ebs.example"]
+}

command/test-fixtures/fmt/unformatted.pkrvars.hcl

@@ -0,0 +1,3 @@
+ami_filter_name ="amzn2-ami-hvm-*-x86_64-gp2"
+ami_filter_owners =[ "137112412989" ]
+