Support priv/unpriv image extraction

Optionally add back privileged extraction which can be enabled with
LUET_PRIVILEGED_EXTRACT=true

Signed-off-by: Ettore Di Giacinto <mudler@sabayon.org>

vendor/github.com/moby/buildkit/source/manager.go

@@ -0,0 +1,49 @@
+package source
+
+import (
+	"context"
+	"sync"
+
+	"github.com/moby/buildkit/cache"
+	"github.com/moby/buildkit/session"
+	"github.com/pkg/errors"
+)
+
+type Source interface {
+	ID() string
+	Resolve(ctx context.Context, id Identifier, sm *session.Manager) (SourceInstance, error)
+}
+
+type SourceInstance interface {
+	CacheKey(ctx context.Context, index int) (string, bool, error)
+	Snapshot(ctx context.Context) (cache.ImmutableRef, error)
+}
+
+type Manager struct {
+	mu      sync.Mutex
+	sources map[string]Source
+}
+
+func NewManager() (*Manager, error) {
+	return &Manager{
+		sources: make(map[string]Source),
+	}, nil
+}
+
+func (sm *Manager) Register(src Source) {
+	sm.mu.Lock()
+	sm.sources[src.ID()] = src
+	sm.mu.Unlock()
+}
+
+func (sm *Manager) Resolve(ctx context.Context, id Identifier, sessM *session.Manager) (SourceInstance, error) {
+	sm.mu.Lock()
+	src, ok := sm.sources[id.ID()]
+	sm.mu.Unlock()
+
+	if !ok {
+		return nil, errors.Errorf("no handler for %s", id.ID())
+	}
+
+	return src.Resolve(ctx, id, sessM)
+}