Packaging the lohrothek as a python module wasn’t really possible until
now because it has been split into separate packages without a single
root and with package names that will quite easily produce name conflicts
(like core, or rest_api).

The core package has now been moved into the lohrothek package namespace
(lohrothek.core). All other modules are now in the lohrothek.modules
package namespace (i.e. lohrothek.modules.audiothek). The
rest_api.frontend package has been renamed to thekno_api
(lohrothek.modules.thekno_api) to better reflect the concrete
implementation.

This change also makes a lot of tooling configuration easier because the
lohrothek package namespace can be configured as single source
entrypoint.