Analyze class structure

[swissspidy]

Extracted from #70:

We might need to move some stuff around. For example delete_local_repository() might make more sense in the Updater instead of Runner.

[swissspidy]

Some more observations after working on #108 and other tickets:

• The Plugin class and especially \Required\Traduttore\Plugin::register_hooks() is getting too complex. We should put all these things into different methods/functions
• A helper method to access the WP_Filesystem API would be handy, e.g. Required\Traduttore\get_filesystem(). We could easily create a inc/namespace.php file with multiple such helper functions.
• Could we combine Updater and Runner without any negative consequences?
• ZipProvider might not be the best name. What about LanguagePack?