Plugin class names consistently stored without a leading slash

A majority of plugins are provided by an annotated class.

When consulting the plugin definition for the class name, it has no leading slash: Drupal\mymodule\MyClass
However, when defining plugins via other means, e.g. YAML, the author can write it either with or without a leading slash.

Since neither the MyClass::class constant nor annotated plugins do not include the leading slash, we should be consistent.

This should have little to no effect on existing code, as checking the plugin class is discouraged.