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/fs.go

@@ -1,32 +1,71 @@
 package plancontext
 
 import (
+	"fmt"
 	"sync"
 
+	"cuelang.org/go/cue"
+	"github.com/google/uuid"
 	bkgw "github.com/moby/buildkit/frontend/gateway/client"
+	"go.dagger.io/dagger/compiler"
+)
+
+var (
+	fsIDPath = cue.MakePath(
+		cue.Hid("_fs", "alpha.dagger.io/dagger"),
+		cue.Str("id"),
+	)
 )
 
 type FS struct {
-	Result bkgw.Reference
+	id     string
+	result bkgw.Reference
+}
+
+func (fs *FS) Result() bkgw.Reference {
+	return fs.result
+}
+
+func (fs *FS) Value() *compiler.Value {
+	v := compiler.NewValue()
+	if err := v.FillPath(fsIDPath, fs.id); err != nil {
+		panic(err)
+	}
+	return v
 }
 
 type fsContext struct {
 	l     sync.RWMutex
-	store map[ContextKey]*FS
+	store map[string]*FS
 }
 
-func (c *fsContext) Register(fs *FS) ContextKey {
+func (c *fsContext) New(result bkgw.Reference) *FS {
 	c.l.Lock()
 	defer c.l.Unlock()
 
-	id := hashID(fs)
-	c.store[id] = fs
-	return id
+	fs := &FS{
+		// FIXME: get a hash from result instead
+		id:     uuid.New().String(),
+		result: result,
+	}
+
+	c.store[fs.id] = fs
+	return fs
 }
 
-func (c *fsContext) Get(id ContextKey) *FS {
+func (c *fsContext) FromValue(v *compiler.Value) (*FS, error) {
 	c.l.RLock()
 	defer c.l.RUnlock()
 
-	return c.store[id]
+	id, err := v.LookupPath(fsIDPath).String()
+	if err != nil {
+		return nil, fmt.Errorf("invalid FS %q: %w", v.Path(), err)
+	}
+
+	fs, ok := c.store[id]
+	if !ok {
+		return nil, fmt.Errorf("fs %q not found", id)
+	}
+
+	return fs, nil
 }