Update libpeas to 1.18.0

* Fix Python plugin loader module paths
* Do not add pyexecdir to Python's paths at runtime
* Add support for embedded C plugins
* Order the PeasEngine:plugin-list by dependencies
* Keep plugins order in PeasExtensionSet
* Add g_autoptr() autocleanup support
* Fix Python tests on OpenBSD
* Fix libpeas crash on windows
* Simplify __all__ support in the Python plugin loader
* Removed support for Seed plugins
* Added support for Lua 5.1 plugins using the LGI bindings
* PeasEngine is now thread-safe
* Added support for non-global plugins loaders
* Load C plugins with local linkage
* Use Python to implement the plugin loader's logic
* Support Python plugins that define __all__
* Use Lua to implement the plugin loader's logic
* Prevent the accidental escaping of globals with Lua plugins
* Misc bugfixes
* Many translation updates

(prlw1)