BUG #15530: unable restore index on function from earthdistancecontrib

The following bug has been logged on the website:

Bug reference:      15530
Logged by:          Sergei Kornilov
Email address:      sk@zsrv.org
PostgreSQL version: 11.1
Operating system:   Linux Debian
Description:

Hello

We have some immutable language SQL functions in earthdistance contrib
module. So we can build indexes on this functions. But it is unable to
restore such indexes from dump due search_path restrictions in pg_dump.

Simple reproducer is:

SELECT pg_catalog.set_config('search_path', '', false);

CREATE EXTENSION IF NOT EXISTS cube WITH SCHEMA public;
CREATE EXTENSION IF NOT EXISTS earthdistance WITH SCHEMA public;

CREATE TABLE public.points (
    id integer NOT NULL,
    name character varying(255) NOT NULL,
    lng double precision NOT NULL,
    lat double precision NOT NULL
);

CREATE INDEX points_coordinates ON public.points USING gist
(public.ll_to_earth(lat, lng));

With error on last line:
> ERROR:  type "earth" does not exist
> LINE 1: ...ians($1))*sin(radians($2))),earth()*sin(radians($1)))::earth
>                                                                  ^
> QUERY:  SELECT

cube(cube(cube(earth()*cos(radians($1))*cos(radians($2))),earth()*cos(radians($1))*sin(radians($2))),earth()*sin(radians($1)))::earth
> CONTEXT:  SQL function "ll_to_earth" during inlining

Seems to be broken in all supported versions. I checked on fresh HEAD, 11.1
and 10.6.
How can we fix this? Extension is marked as relocatable

regards, Sergei