Add an API to describe variant using labels

Add a minimal API that lets plugins process the variant description
and add short labels that could be used to describe the wheel.
The labels are entirely optional -- plugins may not return any,
and clients may ignore them.  The design also assumes that the client
is responsible for choosing how many labels to use before truncating --
though I suppose I'll add a helper function to `variantlib` for that
purpose.

The design assumes that plugin get complete unfiltered
`VariantDescription` -- and therefore also see variant metadata from
other plugins.  I don't think that's really a problem, though it assumes
that the plugin must compare both against provider and key names.

At this point, the protocol makes the API mandatory, even if it would
only return an empty list unconditionally.  Perhaps we should
make the function optional somehow instead.

Author: mgorny

tests/test_plugins.py

@@ -23,7 +23,7 @@ def get_supported_configs(self) -> Optional[ProviderConfig]:
 
     def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
         for meta in variant_desc:
-            if meta.provider == self.__name__ and meta.key == "key1":
+            if meta.namespace == self.namespace and meta.key == "key1":
                 return [meta.value.removeprefix("val")]
         return []
 
@@ -42,17 +42,24 @@ def get_supported_configs(self) -> Optional[ProviderConfig]:
         )
 
     def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
-        if VariantMeta(self.__name__, "key3", "val3a") in variant_desc:
+        if VariantMeta(self.namespace, "key3", "val3a") in variant_desc:
             return ["sec"]
         return []
 
 
 class MockedPluginC(PluginBase):
-    namespace = "incompatible_plugin"
+    namespace = "other_plugin"
 
     def get_supported_configs(self) -> Optional[ProviderConfig]:
         return None
 
+    def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
+        ret = []
+        for meta in variant_desc:
+            if meta.namespace == self.namespace and meta.value == "on":
+                ret.append(meta.key)
+        return ret
+
 
 class ClashingPlugin(PluginBase):
     namespace = "test_plugin"
@@ -63,7 +70,7 @@ def get_supported_configs(self) -> Optional[ProviderConfig]:
     def get_variant_labels(self, variant_desc: VariantDescription) -> list[str]:
         ret = []
         for meta in variant_desc:
-            if meta.provider == self.__name__ and meta.value == "on":
+            if meta.namespace == self.namespace and meta.value == "on":
                 ret.append(meta.key)
         return ret