Rename harvest and datastore to dkan_harvest and dkan_datastore

[dafeder]

These generic names were a bad call. Let's rip off the bandaid and rename them under a dkan_prefix as they are submodules and only (for now) relevant to DKAN.

Acceptance criteria

• Let's avoid any refactoring not directly related to name changes
• Service names should be unaffected
• Use git mv as much as possible to maintain revision history
• Best possible upgrade path. As much as possible handled in update hooks, clear documentation on any manual steps needed.

This almost certainly requires a 3.x release. Let's continue to refine the plan on how we'll manage branches, merge order and release plan.

[dafeder]

Moving this into the epic for renaming, as the deeper reorganization is postponed.