JENKINS-44576 enabled the option to use local cache git repo. Let's extend it and allow to use a directory with a bunch of cloned repos locally. 

It will require backward compatibility:

• CLI option to specify whether to skip the validation of single local repo and allow using a folder with nested cloned repos.
• If such option is specified and the PCT is configured to execute with the localCheckoutPluginDir property, then will copy those cached plugins instead of checking out those projects from SCM, and only if there is a copy of that project locally.