[fud2] Implement Shell Commands With Dependencies

[jku20]

Implements shell commands with dependencies. This has been split off from the origin PR for ease of review.

[jku20]

oops, my git is bad, that didn't quite do what I thought it would

[sampsyo]

Neato! Thanks for separating this out so it was easier to review; it was a lot easier to grok this after understanding #2203.

As you'll see, my big comment is that I don't think that the benefit of allowing mixtures of shell and shell_deps is worth it. It makes the logic pretty weird, it will make the Ninja harder to understand, and it doesn't seem like a feature people will actually want. I think we could trim back the conceptual complexity here if each op had to choose between the two modes of specifying commands.

[jku20]

The previous commit separates out shell and shell_deps functions, making it an error to use both in the same op. This allows for simpler logic when for emitting a RulesOp but requires more complicated logic when building the RhaiOp.

In particular, this implementation tags RhaiOp's cmd field with either being commands from shell, shell_deps, or being empty.

[sampsyo]

Awesome! This all looks great as well.