Support releasing Dafny packages as .NET DLLs

[robin-aws]

This is a high level feature request with a few different related issues/PRs:

• Use Mono.Cecil to read DafnySourceAttribute from DLLs #464 (Stop the dafny CLI from trying to dynamically resolve all dependent DLLs)
• fix: Don't recompile contents of DafnySourceAttribute from DLLs #487 (Needed to support extern code properly)
• Dafny C# runtime should be published as a nuget package #491 (Dafny runtime nuget package)
• fix: Fully-qualify module CompileNames #510 (Otherwise compiled module names are not stable across DLLs)
• Parser should not dynamically inject shared builtin program components #511 (Otherwise leading to errors about trying to define Tuple0 in multiple DLLs)
• Inferred types not qualified in parseable program output #465 (Otherwise reading the Dafny source from a DLL can fail)
• Separate compilation issues #980
• Include dependent DLLs when compiling dafny.msbuild#2 (Make the MSBuild tasks aware of this support and include dependent DLLs when compiling)