The following files for plugin example would go in /mod/example/
actions/
example/
action.php
other_action.php
classes/
VendorNamespace/
PluginNamespace/
ExampleClass.php
languages/
en.php
vendors/
example_3rd_party_lib/
views/
default/
example/
component.css
component.js
component.png
forms/
example/
action.php
other_action.php
object/
example.php
example/
context1.php
context2.php
plugins/
example/
settings.php
usersettings.php
resources/
example/
all.css
all.js
all.php
owner.css
owner.js
owner.php
widgets/
example_widget/
content.php
edit.php
elgg-plugin.php
CHANGES.txt
COPYRIGHT.txt
INSTALL.txt
LICENSE.txt
README.txt
composer.json