Instead initialise all the ones we'll need in advance of building any
packages. The alternative would have been to use a lock, but there's no
particular advantage in trying to delay initialisation and avoiding
locks keeps things simpler.

This should fix #3460

Another similar issue was fixed by 1acc00f8 which serialised the
registration of packages in the local inplace package db, avoiding
lost updates (which could previously be observed in test case T3460).