Consider renaming `boardgame-util` to `boardgame`

[jkomoros]

Pros:

• Shorter, which is useful because the command is run often
• the `//boardgame:codegen comments don't have this weirdness about boardgame-util vs boardgame

Cons

• confusing with boardgame library
• hard to grep for boardgame-util in documentation to change it

Originally captured in #655

[jkomoros]

I think the benefit of it being easy to distinguish from boardgame library in documentation is sufficiently important, and it's not that big of a deal given command auto-completion