Move Extensibility project/package to the right place

The repo dependencies for this package are messed up, leading to weirdness. Extensibility defines common interfaces used by both Script itself as well as our SDK Extensions assemblies. I put the project in the Script repo because it logically belongs there. However that leads to weirdness because the script repo itself depends on this package!

Script correctly treats the package as an external dependency in Script, however it lives in the same repo. So for example, to make a change to that package and pull it in where it needs to go, I first have to make the Extensibility change and check it in so a new package is produced and I can upload to Myget. Then I pull that package in as a reference in Script.

We should fix this.

Start by tracing the Extensibility project/package dependency between the Script repo and the SDK Extensions assemblies. Review how Script references the externally produced package and how the project is currently included in the repository. Done means the package has a consistent location and changes no longer require publishing to MyGet before Script can consume them.

Written by the indexing model from the issue text.