GitHub Releases should only contain source for the specific library

[GitHub Releases](https://github.com/Azure/azure-sdk-for-cpp/releases) should not contain source from the entire repo since we don't guarantee that the entire repo is buildable at any one time.

However, we do know that the specific library the release is for should be buildable at the commit point the release targets. If our GitHub releases only included the source for the one package, it would make a much simpler experience for users looking to acquire the library source in their project.

There also should be no need to include the source for our core libraries, as the specific library's CMake should invoke vcpkg to grab the correct version of all core dependencies.