Allow using requires_none and requires_not_none at the same time

[keith]

This combination can be useful during migrations from old bazel build variables
to new ones. You add your args as requiring the new feature to be not
none, and the old feature to be none.

[lilygorsheneva]

I don't think I understand why these combinations were forbidden in the first place, or why we only want to allow specific ones now.

[keith]

maybe @armandomontanez remembers, i just assume we were being conservative and hadn't tested the full combos

[armandomontanez]

The original reason these were restricted to one per rule was to eliminate confusion around logical behavior of specifying multiple (e.g. OR vs AND). I'm sympathetic to the desire to reduce complexity of some rules by compacting these, but I'd rather that be done as a clearer proposal for how we can prioritize communication of expected behavior to users of the API.

The specific request is:

• Make some test cases to ensure edge cases are understood (e.g. what if requires_none and requires_not_none for the same build variable are set?). I don't remember which of these Bazel provides correctness checks for.
• Propose a way to ensure that users can clearly read the intent as they're parsing the instantiation of these rules in build files.

[keith]

I updated the logic to make sure you don't pass the same var to both, I didn't check what the toolchain logic verifies but i think we'll have a nicer error either way.

I think requires_none + requires_not_none is pretty clear when they can't be the same variable, since it doesn't matter what order anything is evaluated in