Combine common and dkan modules

[dafeder]

#4360 will need to be completed before we can start this work

As a first step in #3746, let's try to move everything in "common" up one level so that we just have a "dkan" module and no "common" module.

Acceptance criteria

• Services that have dkan.common as a prefix should now only have dkan. We can keep the old service names as well for one release cycle but note them as deprecated and internally switch to the new service names.
• No tests should need to be removed, added or significantly changed, just moved into the new appropriate folder and namespaces updated.
• Upgrade path. We must be sure that sites without any custom modules using DKAN services/classes upgrade without any extra steps, and that clear documentation is provided to refactor existing custom code to work with new namespaces and service names.

TBD:

• Can this be released as a minor version release under DKAN 2.x?
• Does it necessitate a 3.x release? Perhaps an alpha/pre-release? Or should it be kept in a 3.x branch until all of #3746 is complete?

[dafeder]

Currently blocked by #4465

[TheETupper]

@paul-m and @dafeder to review together