Fix contexts for user aspects to avoid duplicates.

[vic]

Prevously we had a { OS, HM, host, user} context applied to both, the host aspect and the user aspect, that was causing duplicate definitions since the context was the same and both host and user aspects include den.default.

To fix this properly we now have separate aspects.

The host aspect is applied to { OS, host, user } for each home-managed user.
The user aspect is applied to { HM, host, user } for each home-managed user.

![NOTICE] Update your context-aware aspects if they use { OS, HM, host, user}
As mentioned above this context is now split in two according if it applies to a host or to a user aspect.
I've also updated the documentation: https://den.oeiuwq.com/os-hm-dependencies.html

Also refactored the code to make it more idiomatic and easy to read. Added tests based on the bogus repos from #147 (reply in thread) and #150.

Closes #150