All modules should be loaded when performing post-updates

[pfrenssen]

Fixes #3308.

Postponed on #3310.

[pfrenssen]

The test case replicates the error that was reported by @claudiu-cristea in #3308:

 [notice] Update started: woot_post_update_render
 [error]  The theme implementations may not be rendered until all modules are loaded. 
 [error]  Update failed: woot_post_update_render 

[pfrenssen]

OK this is ready for a look. Note that it is blocked on #3310.

[claudiu-cristea]

I tested this on a complex project with a lot of updates and post-updates and is fixing the bug. We discovered this bug in the same project with the same list of updates/post-updates. I see there's also a test that proves the bug and the fix. @weitzman, @greg-1-anderson, this could be committed but not before #3310.