The generated module is written inside MyREST by default, where a Composer update deletes it

Problem/Motivation

With generated_parent_directory left empty, the generator writes the generated module into the modules directory of MyREST itself and hides it from version control with a .gitignore of its own. On a site that installs MyREST with Composer - the normal way - that directory is replaced whole on every update, so the generated API of the site is deleted by an update and is in no repository the site keeps. The generated module is code of the site and belongs where the site's own modules are.

Proposed resolution

• Default to modules/custom, and refuse a directory inside the MyREST project with a message saying why.
• Stop writing the .gitignore: the generated module is meant to be committed with the site.
• An update path for a site whose generated module already lives inside MyREST: say where it is and how to move it (regenerate after changing the setting).

Remaining tasks

Everything.