Sideload packages for transitions

[lazka]

In case there is a transition which also affects the build tools there are currently two options:

• Include the old DLLs in the new package for a while, until the transition is done
• Bootstrap some packages manually, and let autobuild handle the rest

An alternative would be to upload a special package to autobuild which just includes the old DLLs etc and will be force installed during each build.

This way no old DLLs end up in the repo packages and all packages ending up in the repo are built in autobuild.