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.
Hi Jessie, thanks for the comment. As I understand, the ssh-credential plugin can be used with both Jsch and Trilead SSH implementations, and it's up to the plugin to choose which one to use and redeclare it as non-optional.