Snapcraft doesn't update the `snapcraft-project` disk source when the path is moved

[林博仁(Buo-ren Lin)]

Bug reproduce instructions
--------------------------
1. Run `snapcraft --use-lxd` in one project
1. Move the project directory to another path
1. Re-run `snapcraft --use-lxd`

Current behavior
-----------------
Snapcraft errors with:

```
An error occurred with the instance when trying to start with 'LXD': Common start logic: Missing source '/home/brlin/工作空間/個人專案/Unofficial Snap Packaging for waifu2x' for disk 'snapcraft-project'.
Ensure that 'LXD' is setup correctly and try again.
```

Expected behavior
------------------
Snapcraft should check the project directory path, and if it's changed, update the `snapcraft-project` disk source path