[dotnet cli] Separate command definitions from implementations

[tmat]

The current implementation of dotnet CLI mixes definition of CLI Commands with their implementation (SetAction).

Consequently, code that parses CLI transitively depends on the whole SDK (NuGet, Template Engine, VS Test, etc.).

E.g. dotnet-watch is pulling in a lot of dependencies that it shouldn't. The main reason is that we need to know all existing dotnet commands in order to parse dotnet-watch command line correctly and forward options as needed.

This is also problematic for AOT. We want to be able to do all the parsing inside the AOT 'bubble' and decide on a per-command basis where to call.