hcl2: add example for undefined variable set warn

When a variable is set in a pkrvars file, but isn't defined, an error
message is output, but does not deliver an example of what is expected
by Packer in order to complete a build.

To remedy that, we improve the error message by giving an example of
variable block to include in the build template.

command/build_test.go

@@ -1138,6 +1138,37 @@ func TestBuildCmd(t *testing.T) {
 						nbErrs)
 				}
 
+				return nil
+			},
+		},
+		{
+			name: "hcl - undefined var set in pkrvars",
+			args: []string{
+				testFixture("hcl", "variables", "ref_non_existing"),
+			},
+			expectedCode: 0,
+			outputCheck: func(out, err string) error {
+				if !strings.Contains(out, "Warning: Undefined variable") {
+					return fmt.Errorf("expected 'Warning: Undefined variable' in output, did not find it")
+				}
+
+				nbWarns := strings.Count(out, "Warning: ")
+				if nbWarns != 1 {
+					return fmt.Errorf(
+						"error: too many warnings in build output, expected 1, got %d",
+						nbWarns)
+				}
+
+				if !strings.Contains(out, "variable \"testvar\" {") {
+					return fmt.Errorf("missing definition example for undefined variable")
+				}
+
+				nbErrs := strings.Count(err, "Error: ")
+				if nbErrs != 0 {
+					return fmt.Errorf("error: expected build to succeed without errors, got %d",
+						nbErrs)
+				}
+
 				return nil
 			},
 		},

command/test-fixtures/hcl/variables/ref_non_existing/ref_non_existing_var.pkr.hcl

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

command/test-fixtures/hcl/variables/ref_non_existing/variables.auto.pkrvars.hcl

@@ -0,0 +1 @@
+testvar = "auto"

hcl2template/types.variables.go

@@ -666,11 +666,14 @@ func (cfg *PackerConfig) collectInputVariableValues(env []string, files []*hcl.F
 				diags = append(diags, &hcl.Diagnostic{
 					Severity: sev,
 					Summary:  "Undefined variable",
-					Detail: fmt.Sprintf("A %q variable was set but was "+
-						"not found in known variables. To declare "+
-						"variable %q, place this block in one of your "+
-						".pkr files, such as variables.pkr.hcl",
-						name, name),
+					Detail: fmt.Sprintf("A %[1]q variable was set but was "+
+						"not declared as an input variable. To declare "+
+						"variable %[1]q, place this block in one of your "+
+						".pkr.hcl files, such as variables.pkr.hcl\n\n"+
+						"variable %[1]q {\n"+
+						"  type = string\n"+
+						"}",
+						name),
 					Context: attr.Range.Ptr(),
 				})
 				continue