This refactors the targets used to build the shared framework and its .zip files. There are lots of reasons motivating this: Arcade convergence, migration to VSTS, making it easier to build this locally, etc.
Changes:
* Moves move content of build/Sharedfx.{props/targets} into eng/targets/SharedFx.Common.{props/targets}
* Update the build to produce a `runtime.$rid.Microsoft.AspNetCore.App` package (not just the one with symbols in it)
* Refactor the targets which produce .tar.gz/.zip files into separate projects in `src/Installers/`
* Refactor installers, unit tests, and the framework projects to use ProjectReference to flow dependencies between different parts of the build.
* Makes it easier to build the shared framework locally (for the inner dev loop, you can run `dotnet build -p src/Framework/Microsoft.AspNetCore.App/src/ -r win-x64`)
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