feat(daemon): allow overriding layers directory in daemon.Options

[thp-canonical]

This allows having the layers directory in a different directory than the "layers" sub-directory inside the pebble directory.

The defaults are unchanged, but one can pass an optional non-empty string in daemon.Options{LayersDir: ...} when calling daemon.New() to use a custom location for the layers.

This also removes the need to pass pebbleDir into plan and planstate, as it really only needs the layersDir.

[flotter]

This PR relates to KO064 for supporting a different storage arrangement for a derivative project, where R/W data persisted over reboot is located at a separate location.

[benhoyt]

This looks good to me, thanks. I like how it makes the ReadDir parameter more sensible, too.

[thp-canonical]

@hpidcock What do you think about this change?

[benhoyt]

Harry's busy (and working night) this week with the roadmap virtual sprint. I think this is a reasonable change and well done. Let's just go ahead and merge it.