Migrate dagger.#Secret and dagger.#Stream to new format

- Refactored to keep every transformation of built-in types (e.g. FS,
  Secret, etc) to/from CUE in the same place (plancontext)
- dagger.#Service and dagger.#Secret are now following the new FS-like format
  (e.g. `_service: id: string`)
- Backward compatibility
- dagger.#Stream is now an alias for dagger.#Service

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

plancontext/service.go

@@ -1,27 +1,84 @@
 package plancontext
 
-import "sync"
+import (
+	"fmt"
+	"sync"
+
+	"cuelang.org/go/cue"
+	"go.dagger.io/dagger/compiler"
+)
+
+var (
+	serviceIDPath = cue.MakePath(
+		cue.Hid("_service", "alpha.dagger.io/dagger"),
+		cue.Str("id"),
+	)
+)
+
+type Service struct {
+	id string
+
+	unix  string
+	npipe string
+}
+
+func (s *Service) ID() string {
+	return s.id
+}
+
+func (s *Service) Unix() string {
+	return s.unix
+}
+
+func (s *Service) NPipe() string {
+	return s.npipe
+}
+
+func (s *Service) Value() *compiler.Value {
+	v := compiler.NewValue()
+	if err := v.FillPath(serviceIDPath, s.id); err != nil {
+		panic(err)
+	}
+	return v
+}
 
 type serviceContext struct {
 	l     sync.RWMutex
-	store map[ContextKey]*Service
+	store map[string]*Service
 }
 
-type Service struct {
-	Unix  string
-	Npipe string
-}
-
-func (c *serviceContext) Register(service *Service) ContextKey {
+func (c *serviceContext) New(unix, npipe string) *Service {
 	c.l.Lock()
 	defer c.l.Unlock()
 
-	id := hashID(service)
-	c.store[id] = service
-	return id
+	s := &Service{
+		id:    hashID(unix, npipe),
+		unix:  unix,
+		npipe: npipe,
+	}
+
+	c.store[s.id] = s
+	return s
 }
 
-func (c *serviceContext) Get(id ContextKey) *Service {
+func (c *serviceContext) FromValue(v *compiler.Value) (*Service, error) {
+	c.l.RLock()
+	defer c.l.RUnlock()
+
+	id, err := v.LookupPath(serviceIDPath).String()
+	if err != nil {
+		return nil, fmt.Errorf("invalid service %q: %w", v.Path(), err)
+	}
+
+	s, ok := c.store[id]
+	if !ok {
+		return nil, fmt.Errorf("service %q not found", id)
+	}
+
+	return s, nil
+}
+
+func (c *serviceContext) Get(id string) *Service {
 	c.l.RLock()
 	defer c.l.RUnlock()