[JENKINS-21486] Fix plugin dependencies resolution

* Check that dependencies are enabled. A disabled optional dependency
  will not prevent a plugin from loading.
* Check versions of dependencies declared by a plugin before loading it.
  If any dependency (even optional) is older than what is required,
  then the plugin isn't loaded.

This should prevent use cases where a plugin is loaded but one of its
dependencies is too old so that :
* its @Extension annotated classes cannot be loaded, causing the full
  Jenkins to blow up with crapload of exceptions which are tedious to
  investigate to understand the root cause.
* NoSuchMethodError and the likes at runtime even though boot has
  completed.

Version check (for setups where version list is manually crafted but yet
works) can be disabled by starting Jenkins with

-Dhudson.PluginWrapper.dependenciesVersionCheck.enabled=true

Minor fixes done while implementing this change :
* Fix version parsing in PluginWrapper.Dependency
* Dynamic plugin load didn't check for disabled flag