Completion of nested property of generic source property

还没有人认领这个 Issue。

Example Source class (Lombok used for brevity):
```java
import org.geolatte.geom.G2D;
import org.geolatte.geom.Point;

@Getter
@Setter
public class Location {
private Point coordinates;

// ...
}
```

eg. when attempting to map the latitude of `coordinates`:
```java
@Mapping(source = "coordinates.position.lat", target = "latitude")
```
[Point#getPosition()](https://javadoc.io/static/org.geolatte/geolatte-geom/1.4.0/org/geolatte/geom/Point.html#getPosition--) returns the type argument, in this case `G2D`, but the plugin infers it as `P` (the type parameter), and hence fails to autocomplete the [G2D#getLat()](https://javadoc.io/static/org.geolatte/geolatte-geom/1.4.0/org/geolatte/geom/G2D.html#getLat--) property.

在 IntelliJ IDEA 中复现包含 Location、Point 和 coordinates.position.lat 的嵌套映射示例。首先跟踪插件如何解析 Point#getPosition() 及其泛型类型参数，然后验证自动补全能够到达 G2D#getLat()。当属性被建议为 G2D，而不是未解析的类型参数 P 时，即表示完成。

由索引模型根据 Issue 内容生成。