$ manage dbshell
Password for user testing: 
psql (9.3.6)
Type "help" for help.

project_test=> CREATE TABLE "customers_addresstype" (
project_test(>     "id" serial NOT NULL PRIMARY KEY,
project_test(>     "customer_id" integer NOT NULL REFERENCES "customers_customer" ("id") DEFERRABLE INITIALLY DEFERRED,
project_test(>     "display" citext NOT NULL,
project_test(>     UNIQUE ("customer_id", "display")
project_test(> )
project_test-> ;
ERROR:  type "citext" does not exist
LINE 4:     "display" citext NOT NULL,
                      ^
project_test=> CREATE EXTENSION IF NOT EXISTS citext WITH SCHEMA ext;
ERROR:  could not open extension control file "/usr/share/postgresql/9.3/extension/citext.control": No such file or directory


$ ls /usr/share/postgresql/9.3/extension
plpgsql--1.0.sql  plpgsql.control  plpgsql--unpackaged--1.0.sql

$ ls /usr/lib/postgresql/9.3/lib
ascii_and_mic.so     euc_cn_and_mic.so   latin2_and_win1250.so  pgxs               utf8_and_big5.so      utf8_and_euc_jp.so   utf8_and_gbk.so        utf8_and_sjis2004.so
cyrillic_and_mic.so  euc_jp_and_sjis.so  latin_and_mic.so       plpgsql.so         utf8_and_cyrillic.so  utf8_and_euc_kr.so   utf8_and_iso8859_1.so  utf8_and_sjis.so
dict_snowball.so     euc_kr_and_mic.so   libpqwalreceiver.so    tsearch2.so        utf8_and_euc2004.so   utf8_and_euc_tw.so   utf8_and_iso8859.so    utf8_and_uhc.so
euc2004_sjis2004.so  euc_tw_and_big5.so  pg_upgrade_support.so  utf8_and_ascii.so  utf8_and_euc_cn.so    utf8_and_gb18030.so  utf8_and_johab.so      utf8_and_win.so

and apt-cache search postgres returns

...
postgres-xc - write-scalable, synchronous multi-master, transparent PostgreSQL cluster
postgres-xc-client - front-end programs for Postgres-XC
postgres-xc-contrib - additional facilities for Postgres-XC
postgres-xc-dbg - debug symbols for postgres-xc
postgres-xc-doc - documentation for the Postgres-XC cluster
postgres-xc-plperl - PL/Perl procedural language for Postgres-XC
postgres-xc-plpython - PL/Python procedural language for Postgres-XC
postgres-xc-pltcl - PL/Tcl procedural language for Postgres-XC
postgres-xc-server-dev - development files for Postgres-XC server-side programming
postgresql-9.3-debversion - Debian version number type for PostgreSQL
postgresql-9.3-ip4r - IPv4 and IPv6 types for PostgreSQL 9.3
postgresql-9.3-orafce - Oracle support functions for PostgreSQL 9.3
postgresql-9.3-pgfincore - set of PostgreSQL functions to manage blocks in memory
postgresql-9.3-pgmemcache - PostgreSQL interface to memcached
postgresql-9.3-pgmp - arbitrary precision integers and rationals for PostgreSQL 9.3
postgresql-9.3-pgpool2 - connection pool server and replication proxy for PostgreSQL - modules
postgresql-9.3-pllua - Lua procedural language for PostgreSQL 9.3
postgresql-9.3-plproxy - database partitioning system for PostgreSQL 9.3
postgresql-9.3-plr - Procedural language interface between PostgreSQL and R
postgresql-9.3-plsh - PL/sh procedural language for PostgreSQL 9.3
postgresql-9.3-plv8 - Procedural language interface between PostgreSQL and JavaScript
postgresql-9.3-postgis-2.1 - Geographic objects support for PostgreSQL 9.3
postgresql-9.3-postgis-2.1-scripts - PostGIS for PostgreSQL 9.3 -- scripts -- dummy package
postgresql-9.3-postgis-scripts - Geographic objects support for PostgreSQL 9.3 -- scripts
postgresql-9.3-prefix - Prefix Range module for PostgreSQL
postgresql-9.3-preprepare - pre prepare your PostgreSQL statements server side
postgresql-9.3-repmgr - replication manager support modules for PostgreSQL 9.3
postgresql-9.3-slony1-2 - replication system for PostgreSQL: PostgreSQL 9.3 server plug-in
postgresql-autodoc - Utility to create a PostgreSQL database schema overview in HTML, DOT and XML
postgresql-comparator - efficient PostgreSQL table content comparison and synchronization
postgresql-hll - HyperLogLog extension for PostgreSQL
postgresql-plperl-9.1 - PL/Perl procedural language for PostgreSQL 9.1
postgresql-prioritize - Get and set the nice priorities of PostgreSQL backends
postgresql-server-dev-all - extension build tool for multiple PostgreSQL versions
...

and $ locate -i citext finds nothing.

project_test=> CREATE TABLE "customers_addresstype" (
project_test(>     "id" serial NOT NULL PRIMARY KEY,
project_test(>     "customer_id" integer NOT NULL REFERENCES "customers_customer" ("id") DEFERRABLE INITIALLY DEFERRED,
project_test(>     "display" citext NOT NULL,
project_test(>     UNIQUE ("customer_id", "display")
project_test(> )
project_test-> ;
ERROR:  type "citext" does not exist
LINE 4:     "display" citext NOT NULL,
                      ^
project_test=> CREATE EXTENSION IF NOT EXISTS citext WITH SCHEMA ext;
ERROR:  could not open extension control file "/usr/share/postgresql/9.3/extension/citext.control": No such file or directory


$ ls /usr/share/postgresql/9.3/extension
plpgsql--1.0.sql  plpgsql.control  plpgsql--unpackaged--1.0.sql

$ ls /usr/lib/postgresql/9.3/lib
ascii_and_mic.so     euc_cn_and_mic.so   latin2_and_win1250.so  pgxs               utf8_and_big5.so      utf8_and_euc_jp.so   utf8_and_gbk.so        utf8_and_sjis2004.so
cyrillic_and_mic.so  euc_jp_and_sjis.so  latin_and_mic.so       plpgsql.so         utf8_and_cyrillic.so  utf8_and_euc_kr.so   utf8_and_iso8859_1.so  utf8_and_sjis.so
dict_snowball.so     euc_kr_and_mic.so   libpqwalreceiver.so    tsearch2.so        utf8_and_euc2004.so   utf8_and_euc_tw.so   utf8_and_iso8859.so    utf8_and_uhc.so
euc2004_sjis2004.so  euc_tw_and_big5.so  pg_upgrade_support.so  utf8_and_ascii.so  utf8_and_euc_cn.so    utf8_and_gb18030.so  utf8_and_johab.so      utf8_and_win.so

I don't see in the repository and locate -i citext finds nothing.