plugable: support plugin versioning
Allow multiple incompatible versions of a plugin using the same name. The
current plugins are assumed to be version '1'.
The unique identifier of plugins was changed from plugin name to plugin
name and version. By default, the highest version available at build time
is used. If the plugin is an unknown remote plugin, version of '1' is used
Reviewed-By: David Kupka <email@example.com>