This reverts commit 29c78ce841 and all other fixes
that have been made afterwards.
The motivation is that use-package is seen by many as a replacement for
`require`. Is use-package always defer the loading of packages then is breaks
this use case, this does not respect POLA so even if it was making Spacemacs
loading faster (up to 3s faster on some startup on my machine) we just cannot
use it, it would be irresponsible. Spacemacs should be easy to use, loading
performance will come with time but it is not a priority.
spacemacs/add-flycheck-hook was not really hooking anything, change the name
to better reflect what it does.
Also changed the push for a add-to-list to avoid duplicates.
Flycheck recently gained a feature to explain the error at point (bound
under `C-c ! e` by default). This commit adds a corresponding leader
binding to the flycheck layer.
