Redeclare JSch dependency to the jsch-plugin

So I've created the ssh-plugin and forked it under the jenkinsci/jsch-plugin, this changes the JSch dependency to that plugin. I need this for the ssh-credentials support in ssh-plugin.
This plugin does not do anything with Trilead (that I can see). It does offer authenticators for use with JSch which other plugins may call as an API. Since the proposed `jsch-plugin` offers no UI or any other extensions, there is no reason (that I know of) not to have it installed, so there is no reason to complicate things by making this dependency optional. But @stephenc may have some motivation I am not aware of.