/usr/share/postgresql/10/extension/insert_username--unpackaged--1.0.sql is in postgresql-10 10.3-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 | /* contrib/spi/insert_username--unpackaged--1.0.sql */
-- complain if script is sourced in psql, rather than via CREATE EXTENSION
\echo Use "CREATE EXTENSION insert_username FROM unpackaged" to load this file. \quit
ALTER EXTENSION insert_username ADD function insert_username();
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