Call realpaths for roslaunch and rosbash.

Our Nix-based ros builds create a directory that has each ros package symlinked, so instead of one directory holding all our ros packages, we have one directory holding symlinks to the ros packages. The package move_base would exist in /nix/store/<hash>-sdk/merged/move_base/move_base, but /nix/store/<hash>-sdk/merged/move_base/ would be a symlink to /nix/store/<hash>-move_base/.

If symlinks are not resolved (current behaviour), the execfn in corefiles is the path with the symlink, so the /nix/store/<hash>-sdk/merged/move_base/move_base one. This means that if a developer has to investigate a core file, they need to download all of /nix/store/<hash>-sdk to their local system.

With the symlinks fully resolved (behaviour in this branch), the actual binary started by rosrun or roslaunch is /nix/store/<hash>-move_base/move_base and the developer only has to download /nix/store/<hash>-move_base.

fyi @mikepurvis , @jasonimercer