Simplify package state pinpointing key in plasma-compose.yaml

### Current

- When using a git repo as a package, `tag:`, `ref:` keys can be used to pinpoint a specific state of package
- Using 2 different keys can be misleading / counter-intuitive for developers whi may change the value but forget to fix the key itself


### Expected

- We could merge both `tag` and `ref` to one key `ref` for smoother DX