The public list `hermes plugins validate` checks plugin imports against
was a hardcoded frozenset inside hermes_cli/plugin_validate_private_imports.py,
with no page that showed it to plugin authors.
Move the list, unchanged (89 entries), into hermes_cli/data/plugin_api.json
(already shipped as package data via `data/*.json`) and load it once in the
validator. scripts/gen_plugin_api_docs.py renders
website/docs/developer-guide/plugins/public-api.md from the same file, and a
test fails when the page is stale. Validate output is unchanged for the same
inputs.
