adding the ability for users to override implicitly loaded global libraries.

[willcrain1]

In my company we use this shared pipeline library plugin, and I find it hard to test changes to shared pipeline libraries unless I have write access to their repository and am able to create a branch, then I can refer to the changes with @Library('libraryname@branchname').

Currently it seems there's no way to import a fork of a library to override an implicitly loaded shared library. I'd use this functionality frequently to test any changes made in our shared libraries.

I was able to test this PR by building hpi files and importing them into the jenkinsci official docker container. I then created a shared library here: https://github.com/willcrain1/test-pipeline-library and forked it here as well and made minor changes: https://github.com/jenkinspipelinetesting/test-pipeline-library

I executed the libraries in this Jenkinsfile: https://github.com/willcrain1/test-pipeline/blob/master/Jenkinsfile

@jglick @kohsuke any feedback is appreciated.

[jglick]

As of #172 most of the code in this plugin has been moved to another plugin repository so this PR must be closed. If this change is still needed, please

git clone https://github.com/jenkinsci/pipeline-groovy-lib-plugin
cd pipeline-groovy-lib-plugin
git checkout -b PLEASE-SELECT-BRANCH-NAME
git pull https://github.com/willcrain1/workflow-cps-global-lib-plugin master

resolve any merge conflicts, and file a fresh PR on the new repository. Be sure to paste a link to this old PR to enable bidirectional navigation.