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

まだ誰も着手していません。

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.

まず FooMapper の例で警告を再現し、MapStruct IntelliJ plugin が source が "." である @Mapping の値をどのように処理するかを調べます。その動作を、MapStruct がこの source 形式をサポートしているというドキュメント上の記載と比較します。有効なマッピングに対して誤った警告が表示されなくなるか、警告の理由が明確に特定できれば完了です。

索引モデルが issue の本文から書いたものです。