Howto: Write package dependent code for Unity.

assembly.asmdef

{
  "versionDefines": [
        {
            "name": "com.unity.render-pipelines.high-definition",
            "expression": "7.3.1",
            "define": "USE_HDRP"
        }
    ], 
  }

SomeBehaviour.cs

public partial class SomeBehaviour : MonoBehaviour
{
  //this method can be defined in other file, or just ignored.
  partial public void SomeMethod();
  
  void Update() {
    SomeMethod();
  }
}

SomeBehaviour.HDRP.cs

#if USE_HDRP
public partial class SomeBehaviour : MonoBehaviour
{
  partial public void SomeMethod() {
    
  }
}
#endif