Validation error: Simple names in `safeds.xy` packages must be unique

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

Declarations from `safeds.xy` are imported by default. We must ensure that their simple names are unique.

### Desired solution

Add an appropriate check.

### Possible alternatives (optional)

_No response_

### Screenshots (optional)

_No response_

### Additional Context (optional)

_No response_