Ensure module names and path names are consistent

DUB builds each package separately, and then adds each package to the
import paths using D's usual mechanism of generating the filesystem
path name from the module name.  To make this work, source files must
be in a path that matches the module name.

Create a module at postgres/db.d to avoid breaking existing code that
imports postgres.db. This module prints a deprecation warning and
re-exports the contents of ddb.postgres.