dagger.#FS support

- Implement dagger.#FS support
- Migrate `context.imports` to dagger.#FS
- Backward compat: dagger.#FS can be passed in lieu of a
  dagger.#Artifact
- For instance, an import (`dagger.#FS`) can be passed to the current
  `yarn.#Package` implementation

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

plan/task/secretenv.go

@@ -40,9 +40,8 @@ func (c secretEnvTask) Run(ctx context.Context, pctx *plancontext.Context, _ sol
 		PlainText: env,
 	})
 
-	out := compiler.NewValue()
-	if err := out.FillPath(cue.ParsePath("contents.id"), id); err != nil {
-		return nil, err
-	}
-	return out, nil
+	return compiler.NewValueWithContent(id,
+		cue.Str("contents"),
+		cue.Str("id"),
+	)
 }