Implement support for changing project and package references

[tmat]

Adding a package or project reference requires the following steps:

1. Roslyn validating that the change is allowed (e.g. changing version of preexisting dependency is not allowed since we replace assembly that's already loaded with a new one).
2. Roslyn determines which projects need redeployment and returns the set in ModuleUpdates (along with projects to rebuild/restart).
   Redeploying means copying dependencies that were added via package/project reference. These dlls are not present in output directory and need to be copied there.
   dotnet-watch executes ReferenceCopyLocalPathsOutputGroup target on these projects.
3. At runtime the DotNetDeltaApplier needs to implement AssemblyResolving event that loads the new dependencies. These are not automatically loaded by the runtime since they were not present in the .deps.json file when the app launched.

TODO:

• requires updated roslyn binaries

[dotnet-policy-service]

Thanks for your PR, @@tmat.
To learn about the PR process and branching schedule of this repo, please take a look at the SDK PR Guide.