[JENKINS-33843] Permit disabling optional dependencies (#4001)

* Implied dependencies on detached plugins were not being marked optional despite being listed in optionalDependencies, confusing the calculation of optionalDependents in resolveDependentPlugins.

* Javadoc improvements based on code exploration.

* [JENKINS-33843] Do not consider optional dependencies for purposes of disabling plugins.

* PluginManagerInstalledGUITest updated to reflect new behavior.

(cherry picked from commit 0f73231f)