Update BuildBlock with valid HCL2Ref (#12167)

While looking into #11932 I found that Packer will throw an error when
an unknown source is referenced from within the sources attribute for
a build block. The hcl.Diagnostics error includes a Subject, which
should highlight where in the HCL2 configuration file the unknown source resides.
But when creating the BuildBlock no HCL2Ref data is copied over, thus the
Subject is displayed with with zero value for an hcl.Range type.

This change updates the build creation logic to copy the HCL2Ref data
from the build block at decode time so that it can be properly
referenced downstream.

Closes #11932

Failure due to change in output
```
--- FAIL: TestValidateCommand_ShowLineNumForMissing (0.00s)
    --- FAIL: TestValidateCommand_ShowLineNumForMissing/test-fixtures/validate-invalid/missing_build_block.pkr.hcl (0.00s)
        validate_test.go:377: Unexpected output:   (
                """
                Error: Unknown source file.cho

            -     on  line 0:
            +     on test-fixtures/validate-invalid/missing_build_block.pkr.hcl line 6:
                  (source code not available)

                ... // 4 identical lines
                """
              )
        validate_test.go:379:
FAIL
FAIL    github.com/hashicorp/packer/command     1.002s
```

command/validate_test.go

@@ -1,6 +1,7 @@
 package command
 
 import (
+	"fmt"
 	"path/filepath"
 	"testing"
 
@@ -341,3 +342,42 @@ func TestValidateCommand_VarFilesDisableWarnOnUndeclared(t *testing.T) {
 		})
 	}
 }
+
+func TestValidateCommand_ShowLineNumForMissing(t *testing.T) {
+	tt := []struct {
+		path      string
+		exitCode  int
+		extraArgs []string
+	}{
+		{path: filepath.Join(testFixture("validate-invalid"), "missing_build_block.pkr.hcl"), exitCode: 1},
+	}
+
+	for _, tc := range tt {
+		t.Run(tc.path, func(t *testing.T) {
+			c := &ValidateCommand{
+				Meta: TestMetaFile(t),
+			}
+			tc := tc
+			args := tc.extraArgs
+			args = append(args, tc.path)
+			if code := c.Run(args); code != tc.exitCode {
+				fatalCommand(t, c.Meta)
+			}
+
+			stdout, stderr := GetStdoutAndErrFromTestMeta(t, c.Meta)
+			expected := fmt.Sprintf(`Error: Unknown source file.cho
+
+  on %s line 6:
+  (source code not available)
+
+Known: [file.chocolate]
+
+
+`, tc.path)
+			if diff := cmp.Diff(expected, stderr); diff != "" {
+				t.Errorf("Unexpected output: %s", diff)
+			}
+			t.Log(stdout)
+		})
+	}
+}

hcl2template/types.build.go

@@ -87,22 +87,26 @@ type Builds []*BuildBlock
 // decodeBuildConfig is called when a 'build' block has been detected. It will
 // load the references to the contents of the build block.
 func (p *Parser) decodeBuildConfig(block *hcl.Block, cfg *PackerConfig) (*BuildBlock, hcl.Diagnostics) {
-	build := &BuildBlock{}
-	body := block.Body
-
 	var b struct {
 		Name        string   `hcl:"name,optional"`
 		Description string   `hcl:"description,optional"`
 		FromSources []string `hcl:"sources,optional"`
 		Config      hcl.Body `hcl:",remain"`
 	}
+
+	body := block.Body
 	diags := gohcl.DecodeBody(body, cfg.EvalContext(LocalContext, nil), &b)
 	if diags.HasErrors() {
 		return nil, diags
 	}
 
+	build := &BuildBlock{
+		HCL2Ref: newHCL2Ref(block, b.Config),
+	}
+
 	build.Name = b.Name
 	build.Description = b.Description
+	build.HCL2Ref.DefRange = block.DefRange
 
 	// Expose build.name during parsing of pps and provisioners
 	ectx := cfg.EvalContext(BuildContext, nil)