Rewrite packaging design to reduce maintenance

[cheenamalhotra]

Current build and packaging design is not scalable with the modern .NET SDK packing abilities.
Packing is done as a separate step as we need to sign the binaries, but it requires a separate csproj/nuspec to be created.

In an ideal world, we should:

1. Build the src*.csproj
2. Sign the binaries
3. Pack all packable projects (with --no-build using the same src\csproj for resp. projects)

This should remove the need to maintain packages folder.
More discussion details: #1747 (comment)