There was (and perhaps it exists in other modules, need to check) a bad
habit of importing objects out of module A into B just to be able of
import those object from module B. In this case there was import from
module config_base into module config. This commit fix this.
This is progressing refactor of the calibre code to make it more
readable, and transform it to something more coherent.
In this patch, there are changes regarding imports for some modules,
instead of polluting namespace of each module with some other modules
symbols, which often were imported from other modules. Yuck.
