Plugin definitions should store their class in a consistent manner (without leading slashes)

Problem/Motivation

Annotated classes will never have a leading slash, but putting class: '\Drupal\mymodule\MyClass' in a YAML-based definition is also valid.
Ideally the class itself wouldn't be compared by any code, but it is done in contrib, and should work consistently.

When using the MyClass::class constant, no leading slash is included.

Proposed resolution

Ensure plugin class names do not have leading slashes everywhere.

Remaining tasks

N/A

User interface changes

API changes

Data model changes