Make sure we always create the /public folder

Make sure we always create the /public folder

* Before this commit, when you had static files in the root of /content and no /public folder, that folder would not be created unless the /static syncer had already run.
* So, with a common pattern doing `rm -rf public && hugo` would the fail now and then because /static and /content are processed in parallel (unless you have cleanDestinationDir=true)
* This was even worse before commit 0b918e131fe523188b820d1e3fa0b08251abde69 – a frozen build.

Closes #8166