Including SQL files

I am programming a word game backend in PL/pgSQL and have already reached a point, where (too) many stored functions are declared in a single file words.sql:

# SELECT proname || '(' || oidvectortypes(proargtypes) || ')'
        FROM pg_proc INNER JOIN pg_namespace ns ON (pg_proc.pronamespace = ns.oid)
        WHERE ns.nspname = 'public' ORDER BY proname;
                            ?column?
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 test1(integer)
 words_hash(character varying)
 words_join_new_game(integer, integer)
 words_merge_users(jsonb, inet)
 words_paying_user(integer)
 words_play(integer, integer, jsonb)
 words_shuffle(character varying[])
 words_skip_game(integer, integer)
 words_surrender_game(integer, integer)
 words_swap_game(integer, integer, character varying)
 words_trigger()
 words_valid_user(integer, character varying, character varying)
(12 rows)