Write generated files into a separate site module

Problem/Motivation

Generated code is specific to the site it was generated for, and this module's repository is published on drupal.org. Writing generated plugins inside the module would put one site's content model into a public release, and would make the module's own repository dirty after every scan.

Proposed resolution

• The generator writes into a neighbouring module — myrest_generated by default, the name configurable — creating it if it does not exist, with its info file and namespace.
• The target directory is configurable and validated as writable before anything is written; a read-only filesystem must produce a clear message, not a half-written module.
• Writes are atomic per run: either the whole set lands or nothing does, so an interrupted generation cannot leave a site with half a routing table.
• The module ships with a README saying what it is, that it is generated, and where to put custom code instead.

Remaining tasks

Everything.