Put each plugin in its own folder under plugins/<name>/ with an __init__.py.
Keep plugin-specific assets beside the code in that same folder.
Single-file .py plugins are still supported, but package folders are the recommended plug-and-play format.

That means a plugin can ship as a drag-and-drop folder with extras such as:

cookies.txt
templates or fixture files
helper modules
small static assets

Built-in bundled plugins use the same layout as external plugins. Additional drop-in plugin search paths are:

plugins/ in the repo root
plugins/ in the current working directory
Any directory listed in MM_PLUGIN_PATH
Any directory listed in MEDEIA_PLUGIN_PATH

Plugin rules:

A plugin can be a single .py file or a package directory with __init__.py.
Define a class that inherits from ProviderCore.base.Provider.
Give it a stable name using PLUGIN_NAME or the class name.

Example skeleton:

from ProviderCore.base import Provider, SearchResult


class MyPlugin(Provider):
    PLUGIN_NAME = "myplugin"
    URL_DOMAINS = ("example.com",)

    def search(self, query, limit=50, filters=None, **kwargs):
        text = str(query or "").strip()
        if not text:
            return []
        return [
            SearchResult(
                table="myplugin",
                title=f"Result for {text}",
                path=f"https://example.com/{text}",
            )
        ]

Bundled walkthrough:

The repo now includes a real FTP example plugin in plugins/ftp/__init__.py.
The walkthrough is in docs/ftp_plugin_tutorial.md and shows search-file -plugin ftp, folder drill-in via @N, file download routing, @N | add-file -store ..., and add-file -plugin ftp uploads.
The repo also includes an SCP example plugin in plugins/scp/__init__.py.
The walkthrough is in docs/scp_plugin_tutorial.md and shows search-file -plugin scp, SSH-backed directory drill-in, file download routing, @N | add-file -store ..., and add-file -plugin scp uploads.
The repo now also includes a built-in HydrusNetwork provider in plugins/hydrusnetwork/__init__.py. It delegates to configured store.hydrusnetwork.* backends so Hydrus features can be reached through the normal plugin registry without cmdlets importing Hydrus modules directly.