Adjust things so that instead of creating the store package db during
planning, we instead create them at the beginning of the building phase
along with the other package dbs we create at that point. That is,
create it in a write operation rather than in what is morally a read
operation.

Originally we had to do it in the planning phase because we used to read
the compiler package db to see what was installed, whereas now we just
use the store dir listing.

In fact we were already creating some package dbs during the building
phase, just not all of them, so this cleans things up in that respect.