Remove pkg/backend package

We no longer use registration pattern for backends, as it is using
globals and init() which we want to avoid using.

We also moved the interface to cli/cmd/cluster package, as it was
the only consumer of it, so actually the entire package can be now
removed, as it no longer has any use.

Refs #992

Signed-off-by: Mateusz Gozdek <mateusz@kinvolk.io>

pkg/backend/backend.go

@@ -15,8 +15,6 @@
 package backend
 
 import (
-	"fmt"
-
 	"github.com/hashicorp/hcl/v2"
 )
 
@@ -29,28 +27,3 @@ type Backend interface {
 	// Validate validates backend configuration.
 	Validate() error
 }
-
-// backends is a collection in which all Backends get automatically registered.
-var backends map[string]Backend
-
-// Initialize package's global variable on import.
-func init() {
-	backends = make(map[string]Backend)
-}
-
-// Register registers Backend b in the internal backends map.
-func Register(name string, b Backend) {
-	if _, exists := backends[name]; exists {
-		panic(fmt.Sprintf("backend with name %q registered already", name))
-	}
-	backends[name] = b
-}
-
-// GetBackend returns the Backend referred to by name.
-func GetBackend(name string) (Backend, error) {
-	backend, exists := backends[name]
-	if !exists {
-		return nil, fmt.Errorf("no backend with name %q found", name)
-	}
-	return backend, nil
-}

pkg/backend/local/local.go

@@ -19,7 +19,6 @@ import (
 	"github.com/hashicorp/hcl/v2/gohcl"
 
 	"github.com/kinvolk/lokomotive/internal/template"
-	"github.com/kinvolk/lokomotive/pkg/backend"
 )
 
 type local struct {
@@ -32,11 +31,6 @@ const (
 	Name = "local"
 )
 
-// init registers local as a backend.
-func init() {
-	backend.Register(Name, NewConfig())
-}
-
 // LoadConfig loads the configuration for the local backend.
 func (l *local) LoadConfig(configBody *hcl.Body, evalContext *hcl.EvalContext) hcl.Diagnostics {
 	if configBody == nil {

pkg/backend/s3/s3.go

@@ -22,7 +22,6 @@ import (
 	"github.com/hashicorp/hcl/v2/gohcl"
 
 	"github.com/kinvolk/lokomotive/internal/template"
-	"github.com/kinvolk/lokomotive/pkg/backend"
 )
 
 type s3 struct {
@@ -39,11 +38,6 @@ const (
 	Name = "s3"
 )
 
-// init registers s3 as a backend.
-func init() {
-	backend.Register(Name, NewConfig())
-}
-
 // LoadConfig loads the configuration for the s3 backend.
 func (s *s3) LoadConfig(configBody *hcl.Body, evalContext *hcl.EvalContext) hcl.Diagnostics {
 	if configBody == nil {