Build variant and task names should not be allowed to have commas, or be called "*" or "all"

We currently only ban bars (|) in the names of build variants and tasks. would be ambiguous in dependencies which use *, which means "all tasks" or "all build variants". We should also warn for commas or "all" in tasks and build variants, because they would be ambiguous in the CLI invoked through a shell (because we allow users to specify comma-separated lists of tasks and build variants and "all" is a magic keyword for running all variants or all tasks).