Split plugin package into `store` and `v2/plugin`. Now the functionality
is clearly delineated:
- Manager: Manages the global state of the plugin sub-system.
- PluginStore: Manages a collection of plugins (in memory and on-disk)
- Plugin: Manages the single plugin unit.
This also facilitates splitting the global PluginManager lock into:
- PluginManager lock to protect global states.
- PluginStore lock to protect store states.
- Plugin lock to protect individual plugin states.
Importing "github.com/docker/docker/plugin/store" will provide access
to plugins and has lesser dependencies when compared to importing the
original monolithic `plugin package`.
Signed-off-by: Anusha Ragunathan <anusha@docker.com>
