Allow other files to be imported inside migrations

[d4sein]

A feature similar to psql's \include, where you give a path to a file and the migration runs it as though it had been typed there. This would greatly benefit teams that separate their functions and data types from migrations.

An example of what I think would be cool:

-- migrate:up

CREATE TABLE core.internal_user (
  user_id    BIGINT DEFAULT internal.generate_snowflake_id() PRIMARY KEY,
  created_at TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP
);


\i functions/api/create_user_id.sql
\i functions/api/get_user_id.sql


-- migrate:down

Thanks!

Edit: Realized this wouldn't work because clean migrations would inject the latest version of files and it would mess up dependencies..