Make Nightly builds possible in Boards Manager

Hey guys!
@igrr was investigating the possibility to have nightly packages for the ESP8266 and ESP32 and found out that it is currently a bad idea/not possible.
Here is a link to the issue on our side: https://github.com/esp8266/Arduino/pull/2936
Do you guys have any ideas how we can do that? I do not mind putting the time to modify and PR any changes required to the package manager, but I am not intimate with the code yet and do not really know how it works.
Basically I'm asking if we can make the package manager figure out that there are earlier versions of given package based on the folder name or package manifest, rather than expecting to have the information for it in the new manifest

Start by reading the linked ESP8266 Arduino PR and tracing the Boards Manager package-manager code that handles package folders and manifests. Determine how earlier package versions are currently discovered; done means nightly ESP8266 and ESP32 packages can be recognized without requiring their version information in the new manifest.

Written by the indexing model from the issue text.