Encode mutually-incompatible pairs of markers (#9444)
## Summary
This is an alternative to #9344. If accepted, I need to audit the
codebase and call sites to apply it everywhere, but the basic idea is:
rather than encoding mutually-incompatible pairs of markers in the
representation itself, we have an additional method on `MarkerTree` that
expands the false-y definition to take into account assumptions about
which markers can be true alongside others. We then check if the the
current marker implies that at least one of them is true.
So, for example, we know that `sys_platform == 'win32'` and
`platform_system == 'Darwin'` are mutually exclusive. When given a
marker expression like `python_version >= '3.7'`, we test if
`python_version >= '3.7'` and `sys_platform != 'win32' or
platform_system != 'Darwin'` are disjoint, i.e., if the following can't
be satisfied:
```
python_version >= '3.7' and (sys_platform != 'win32' or platform_system != 'Darwin')
```
Since, if this can't be satisfied, it implies that the left-hand
expression requires `sys_platform == 'win32'` and `platform_system ==
'Darwin'` to be true at the same time.
I think the main downsides here are:
1. We can't _simplify_ markers based on these implications. So we'd
still write markers like `sys_platform == 'win32' and platform_system !=
'Darwin'`, even though we know the latter expression is redundant.
2. It might be expensive? I'm not sure. I don't think we test for
falseness _that_ often though.
Closes #7760.
Closes #9275.
