uv-pep508: add more methods for simplifying extra-related expressions

[BurntSushi]

In the course of working on #9289, I've had to devise some additions to
our markers. While we are still staying strictly compatible with the
PEP 508 format, we will be abusing the extra expression to carry a
lot more information.

Specifically, we want the following additional operations:

• Simplify extra != 'foo'
• Remove all extra expressions
• Remove everything except extra expressions

My work on #9289 requires all of these (which will be in a future in
PR).

This PR also allows the if_not_else Clippy lint. Specifically, the
lint was flagging this code:

        if !matches!(node.var, Variable::Extra(_)) {
            i = NodeId::FALSE;
            for child in node.children.nodes() {
                i = self.or(i, child.negate(parent));
            }
            if i.is_true() {
                return NodeId::TRUE;
            }
            self.only_extras(i)
        } else {
            // Restrict all nodes recursively.
            let children = node.children.map(i, |node| self.only_extras(node));
            self.create_node(node.var.clone(), children)
        }

It wants you to flip the if and else bodies and un-negate the
condition. But I wanted to keep it as-written because I find it easier
to read. And I think this applies more generally. I think there are
some cases where un-negating a condition can be easier to read (in
particular, double negatives), but I don't think it applies universally
to the point where we should lint against it.

[konstin]

I'm trying to grok what this function does. What would it do with ((os_name == ... and extra == foo) or (sys_platform == ... and extra != foo))?

[BurntSushi]

It would give you os_name == ... or sys_platform == ... back. Basically, it removes all extra nodes by assuming they are true.

[BurntSushi]

I added this as an example to the docs of this function.