Consider QoL improvements to tar artifacts generated by kit

[amisevsk]

Describe the problem you're trying to solve
When packing a modelkit, each layer consists of a tar of the files in that layer's directory. For example, packing a model layer

model:
  path: model-files/

will result in a plain tar containing the contents of model-files (but not the directory itself). During unpacking, this context is reconstructed from the Kitfile so that digests are unchanged.

In addition, creating reproducible modelkits (unpacking and repacking results in the same digest) requires some additional changes, such as setting owner/group, timestamps, etc. to a known value.

This means that the tar files generated by Kit lose some information from the packing environment -- tooling using modelkits depends on kit-specific logic for extracting the contents of a modelkit.

Describe the solution you'd like
To make layer tars more useful outside of a kit context, it would be useful to find a way to include more of the context from the packing environment (e.g. directory names).

Additional context
Considering this change, it's likely something we want to decide sooner rather than later; any change to the pack process will break reproducibility of repacking unpacked modelkits. We also need to consider compatibility, so that previously-packed modelkits can still be unpacked with whatever new format we choose.

[bmicklea]

If implemented these will be breaking changes so better to do sooner than later. We'll take it in the sprint to discuss how to handle and (maybe) to implement.

[bmicklea]

Sprint 17 goal: make the decision on what we are going to do for this.

Implement the change can follow in a future sprint if needed.