Copying sources next to the binary allows gdb to automatically detect
the sources. One still has to start up gdb from this directory though.

Follow up to #12294

1. 1. Before this patch one needs to:

```
cd target/getting-started-1.0-SNAPSHOT-native-image-source-jar
gdb ../getting-started-1.0-SNAPSHOT-runner
```

To make `gdb` detect the sources without the need of executing gdb's `directory` or `set directories` commands.

1. 1. After this patch one needs to:

```
cd target
gdb getting-started-1.0-SNAPSHOT-runner
```

which should be more intuitive.

Since this is not a major improvement we might as well skip it.