Remove containerd specific parts of the plugin package to prepare its
move out of the main repository. Separate the plugin registration
singleton into a separate package.
Separating out the plugin package and registration makes it easier to
implement external plugins without creating a dependency loop.
Signed-off-by: Derek McGowan <derek@mcg.dev>
The plugins packages defines the plugins used by containerd.
Move all the types and properties to this package.
Signed-off-by: Derek McGowan <derek@mcg.dev>
