When a `<Reference>` is named `*.Sources`, set PrivateAssets=All (exclude from generated nuspec) and IncludeAssets=ContentFiles (only consume content files, not .dll's)
Part of #4246
Changes:
* Update source code layout to follow the new conventions for this repo
* Update project files to use `<Reference>`
* Update targets to build NPM packages
* Update BuildTools to support custom 'restore' and 'test' targets
Mikael Mengistu
BrennanConroy
Nate McMaster
Nate McMaster