review-tools error message about external symlinks should show the targets

[Ian Johnson]

It would be helpful to know not just that there are external symlinks pointing to outside of the snap but also where they are pointing to be able to fix it. For example with absolute symlinks where they should be fixed to just be relative, it would be helpful if review-tools also output the specific target of those symlinks. This is the current output for example:

```
 - lint-snap-v2:external_symlinks
 package contains external symlinks: usr/lib/aarch64-linux-gnu/libhistory.so, usr/lib/aarch64-linux-gnu/libreadline.so, usr/lib/aarch64-linux-gnu/libtinfo.so, usr/lib/aarch64-linux-gnu/libuuid.so
```

All of these files are absolute symlinks pointing to the versioned .so file, so it would be easy for folks to see that these just need to become relative in the same directory to fix the problem.

```
$ readlink ./usr/lib/aarch64-linux-gnu/libuuid.so
/lib/aarch64-linux-gnu/libuuid.so.1.3.0
$ readlink ./usr/lib/aarch64-linux-gnu/libtinfo.so
/lib/aarch64-linux-gnu/libtinfo.so.6
$ readlink ./usr/lib/aarch64-linux-gnu/libreadline.so
/lib/aarch64-linux-gnu/libreadline.so.8
$ readlink ./usr/lib/aarch64-linux-gnu/libhistory.so
/lib/aarch64-linux-gnu/libhistory.so.8
```