Validation error: Only stub and test files must have `safeds.xy` package

### Is your feature request related to a problem?

The `safeds.xy` package is reserved for builtin declarations. It should not be used to define segments or pipelines.

### Desired solution

Add an appropriate check.

### Possible alternatives (optional)

_No response_

### Screenshots (optional)

_No response_

### Additional Context (optional)

_No response_