Allow single constraint config setting to be used as modifier

Summary: For buck1 compatibility reasons, single constraint setting config settings are everywhere in the repo. For example, `ovr_config//os:linux` is a config setting for constraint `ovr_config//os/constraints:linux`. Allow single-constraint config settings to be used as modifiers as wel so we can specify `ovr_config//os:linux` as a modifier

Reviewed By: stepancheg

Differential Revision: D51292953

fbshipit-source-id: 6da6b4993c6fa2d3d700f1f5126f25d9d5c94b58

cfg/experimental/common.bzl

@@ -120,7 +120,15 @@ def get_modifier_info(
             selector = modifier_selector_info,
         )
     if isinstance(modifier, str):
-        constraint_value_info = refs[modifier][ConstraintValueInfo]
+        modifier_info = refs[modifier]
+        if ConstraintValueInfo in modifier_info:
+            # In practice, every target with a ConstraintValueInfo also has a ConfigurationInfo,
+            # so this is just a small optimization for targets with ConstraintValueInfo.
+            constraint_value_info = modifier_info[ConstraintValueInfo]
+            return constraint_value_info.setting.label, constraint_value_info
+        cfg_info = modifier_info[ConfigurationInfo]
+        asserts.true(len(cfg_info.constraints) == 1, "Modifier should only be a single constraint value. Found multiple in `{}`".format(modifier))
+        constraint_value_info = list(cfg_info.constraints.values())[0]
         return constraint_value_info.setting.label, constraint_value_info
     fail("Internal error: Found unexpected modifier `{}` type `{}`".format(modifier, type(modifier)))