Refactor op/script/component loading and spec validation

Signed-off-by: Solomon Hykes <sh.github.6811@hykes.org>

dagger/types.go

@@ -2,16 +2,35 @@ package dagger
 
 import (
 	"context"
+	"fmt"
+	"os"
 
 	cueflow "cuelang.org/go/tools/flow"
 )
 
+var ErrNotExist = os.ErrNotExist
+
 // Implemented by Component, Script, Op
 type Executable interface {
 	Execute(context.Context, FS, *Fillable) (FS, error)
 	Walk(context.Context, func(*Op) error) error
 }
 
+func newExecutable(v *Value) (Executable, error) {
+	// NOTE: here we need full spec validation,
+	//   so we call NewScript, NewComponent, NewOp.
+	if script, err := NewScript(v); err == nil {
+		return script, nil
+	}
+	if component, err := NewComponent(v); err == nil {
+		return component, nil
+	}
+	if op, err := NewOp(v); err == nil {
+		return op, nil
+	}
+	return nil, fmt.Errorf("value is not executable")
+}
+
 // Something which can be filled in-place with a cue value
 type Fillable struct {
 	t *cueflow.Task