nixpkgs: reorganize the postgresql extensions

This is a backwards compatible change; it mostly puts all the extensions
for postgresql in a common directory to keep them isolated.

It also moves a few things that /were not/ extensions out into other parts of
the filesystem namespace; namely the postgresql_jdbc and psqlodbc libraries
were moved under development/java-modules and development/libraries,
respectively. Because these libraries use the libpq postgresql client drivers,
they're less sensitive to underlying version changes anyway (since the protocol
is relatively stable).

No attributes were renamed or harmed in the creation of this patch.

Signed-off-by: Austin Seipp <aseipp@pobox.com>