Switch to go-multierror

It seems there are 4 locations left that use the `helper/multierror`
package, where the rest is TF settled on the `hashicorp/go-multierror`
package.

Functionally this doesn’t change anything, so I suggest to delete the
builtin version as it can only cause confusion (both packages have the
same name, but are still different types according to Go’s type system.

Author: Sander van Harmelen

builtin/providers/aws/config.go

@@ -5,7 +5,7 @@ import (
 	"log"
 	"strings"
 
-	"github.com/hashicorp/terraform/helper/multierror"
+	"github.com/hashicorp/go-multierror"
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"

builtin/providers/aws/resource_aws_db_security_group.go

@@ -9,8 +9,8 @@ import (
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/awserr"
 	"github.com/aws/aws-sdk-go/service/rds"
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/helper/hashcode"
-	"github.com/hashicorp/terraform/helper/multierror"
 	"github.com/hashicorp/terraform/helper/resource"
 	"github.com/hashicorp/terraform/helper/schema"
 )

builtin/providers/heroku/resource_heroku_app.go

@@ -5,7 +5,7 @@ import (
 	"log"
 
 	"github.com/cyberdelia/heroku-go/v3"
-	"github.com/hashicorp/terraform/helper/multierror"
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/helper/schema"
 )
 

config/config.go

@@ -8,10 +8,10 @@ import (
 	"strconv"
 	"strings"
 
+	"github.com/hashicorp/go-multierror"
 	"github.com/hashicorp/terraform/config/lang"
 	"github.com/hashicorp/terraform/config/lang/ast"
 	"github.com/hashicorp/terraform/flatmap"
-	"github.com/hashicorp/terraform/helper/multierror"
 	"github.com/mitchellh/mapstructure"
 	"github.com/mitchellh/reflectwalk"
 )