Do not generalize uniquene, only for Downloader

That is the only component that we effectively need to ensure the
uniqueness of the elements. The code was simplified to meet the
new design.
Fix total processed requirements calculation

There two main problems causing this miscalculation:

 1) Non-normalized requirements were getting duplicated;

 2) We were not counting the requirements discarded by the downloader;
Fix messed up debug msg
Rename update_count() to update_count_and_data() in the installer
Remove the last bits of Maestro that touches the main flow

The maestro component is not responsible for anything in the main flow
anymore. Now that we know that the dependencer will always yield the
right data, we just relay on it to tell us which requirements we should
aggregate to install;

The Mapping (the old Maestro) is only present in the `load_installer()`
method of the `Install` command, merging the requirements into package
names and finding the best version among all collected packages.