Unexpected warning: ''.'' should not be used as a source

Nadie ha tomado este issue todavía.

I often have a use-case when `"."` is a valid and correct source of my mapping. It is officially supported mapping source, so why the plugin reports it as "should not be used"? Is there any particular reason or maybe it is just a historical burden from the times when MapStruct actually didn't support `"."` as a source?

Example:

```java
record Failure(String code, String detail) {}

class Foo {
Failure failure;
}

class FooDto {
String failureCode;
String failureDetail;
}

public abstract class FooMapper {

@Mapping(target = "failure", source = ".")
public abstract Foo fromDto(FooDto fooDto);

protected Failure mapFailure(FooDto fooDto) {
return new Failure(fooDto.getFailureCode(), fooDto.getFailureDetail());
}

}
```

Of course, in this particular case we can just use:

```java
@Mapping(target = "failure.code", source = "failureCode")
@Mapping(target = "failure.detail", source = "failureDetail")
```

But there are more complex cases when the target object should be created manually, by a custom code, and the only possible source is the whole source object.

Empieza reproduciendo la advertencia con el ejemplo de FooMapper e inspecciona cómo el MapStruct IntelliJ plugin gestiona los valores de @Mapping cuyo source es ".". Compara ese comportamiento con el soporte documentado de MapStruct para esta forma de source; se considera terminado cuando la asignación válida ya no genere una advertencia incorrecta o se haya establecido claramente el motivo de la advertencia.

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