depends_on fails across stack manifests

[spearki]

Describe the bug
In a stack defined with multiple docker-compose manifests, depends_on fails if a service in one manifest depends on a service in a different manifest.

This seems to be because depends_on is validated during unmarshalling:

okteto/pkg/model/stack_serializer.go

Line 273 in 1474c7a

if err := validateDependsOn(s); err != nil {

To Reproduce
Steps to reproduce the behavior:

db.yml:
  services:
    db:
      image: mariadb

app.yml:
  services:
    app:
      image: nginx
      depends_on:
      - db

$ okteto stack deploy -f db.yml -f app.yml
 x   Service 'app' depends on service 'db' which is undefined.

Expected behavior
depends_on should work across files.