nfpm: support chaining multiple InjectNfpmPackageFieldsRequests (sorted w/ simple priority integer)

[cognifloyd]

This extends then pants.backend.experimental.nfpm backend's plugin API, allowing multiple implementations of this polymorphic-rule plugin hook (before this change, only one implementation was allowed):

• inject_nfpm_package_fields(InjectNfpmPackageFieldsRequest) -> InjectedNfpmPackageFields

Unlike many polymorphic/union rules, each rule runs sequentially instead of concurrently. This is conceptually similar to middleware, because the request passed to each rule includes the results of the previous (lower priority) rule. So, one rule can override or extend fields that were injected in previous rules, not just the original fields from the BUILD-file-based nfpm_*_package target.

I tried several approaches to sorting the request classes, but most were error prone or simply didn't work. Heuristic-based sorting proved to be complex and didn't work in all of my test cases (eg one heuristic was looking at the class's module to make pants-provided requests lower priority). A simple priority integer is far simpler and more reliable, so that's what this implements.

Another thing that didn't work was adding __lt__ as a @classmethod on the abstract InjectNfpmPackageFieldsRequest. That fails because sorted() requires an instance method, not a class method. The only way I found to add an __lt__ method on a class is via a metaclass. So, this adds a _PrioritizedSortableClassMetaclass to InjectNfpmPackageFieldsRequest allowing for simple OneRequest < TwoRequest sorting of the class type like this:

     inject_nfpm_config_request_types = union_membership.get(InjectNfpmPackageFieldsRequest)
     applicable_inject_nfpm_config_request_types = tuple(
-        request
-        for request in inject_nfpm_config_request_types
-        if request.is_applicable(target)
+        sorted(
+            request_type
+            for request_type in inject_nfpm_config_request_types
+            if request_type.is_applicable(target)
+        )
     )

Then, to make subclasses higher priority than their parent classes, I added an __init_subclass__ method that increases cls.priority (unless the subclass already has a different priority than the parent class). It's a little odd to use both a metaclass and __init_subclass__, but I did not like the idea of re-implementing the on-subclass-creation logic in our metaclass.

InjectNfpmPackageFieldsRequest was initially modeled on the python backend's SetupKwargsRequest. If the rule-chaining proves itself in the experimental nfpm backend, maybe we can move _PrioritizedSortableClassMetaclass out of the backend to somewhere in pants core, updating SetupKwargsRequest and related rules to make use of rule-chaining as well.

[cognifloyd]

I extracted this PR from #22861