Plugin manager refactoring and internal cleanups

This issue tries to sum up the different issues regarding plugin refactoring, internal cleanups the current open tasks/issues are:
- [ ] Plugin install --development should be sticky #2631
- [x] refactor of pluginmanager/utils.rb #3746
- [ ] Upgrade validation of validate_major_version #3744 #3818
- [ ] Logging in the plugin manager #3656 #2627
- [ ] Lock file to prevent concurrent access to the Gemfile #2622
- [ ] Only install logstash plugins with the plugin manager #2047

Start with pluginmanager/utils.rb and review the referenced issues #2631, #3744, #3818, #3656, #2627, #2622, and #2047. The work is complete only when the remaining unchecked plugin-manager tasks have been resolved and the checklist accurately reflects their status.

Written by the indexing model from the issue text.