Adjust the methods dealing with $_GET and $_POST requests to be able to deal with updates as well

Most significant changes:
- Add if/else switch code to deal with both install as well as update requests
- Introduced new `inject_update_info()` method which adjusts the update_plugins transient to contain information on external and pre-packaged plugins as WP checks the transient before executing an update
- Introduced new `get_plugins_api()` method to retrieve information from the wp.org repo API and apply some minimalistic caching for that.
- Re-evaluated what information really needs to be passed to the Upgrader classes and how and implement this properly.
- Updated the `maybe_adjust_source_dir()` to deal with the changes in passed information
- Only update a plugin if the new version is compatible with the WP version installed

Author: jrfnl