Use buck transclusion to simplify constraint_overrides.bzl

Summary:
This simplifies `constraint_overrides.bzl` to only depend on single configs for the input list of allowed `constraint_overrides` and `platform_overrides`.

Manually maintained configs are renamed for clarity do distinguish them from the generated configs (and because `$(config)` cannot reference the same config it's modifying).

BUCK transclusion: https://buck2.build/docs/concepts/buckconfig/#transclusion-of-values-from-one-key-to-another

Reviewed By: scottcao

Differential Revision: D68281409

fbshipit-source-id: 448ca988da7319a40ccb146428397c2f338b11cf

Author: Tarkan Al-Kazily

prelude/transitions/constraint_overrides.bzl

@@ -8,17 +8,17 @@
 # NOTE: Currently, constraints can't be propagated via rule attrs and so need to be
 #       hard-coded here. We use a read_config to avoid hard-coding these repo-specific
 #       constraints into the prelude.
+#       The following configs must be set to use constraint_overrides.bzl:
+#         - buck2.platforms
+#         - buck2.constraints
+#         - buck2.passthrough_constraints
 
 def _platforms() -> list[str]:
     platforms = []
     config = read_root_config("buck2", "platforms", "")
     platforms.extend(
         [platform.strip() for platform in config.split(",") if platform.strip()],
     )
-    config = read_root_config("buck2", "generated_platforms", "")
-    platforms.extend(
-        [platform.strip() for platform in config.split(",") if platform.strip()],
-    )
     return platforms
 
 def _constraints() -> list[str]:
@@ -27,10 +27,6 @@ def _constraints() -> list[str]:
     constraints.extend(
         [constraint.strip() for constraint in config.split(",") if constraint.strip()],
     )
-    config = read_root_config("buck2", "generated_constraints", "")
-    constraints.extend(
-        [constraint.strip() for constraint in config.split(",") if constraint.strip()],
-    )
     return constraints
 
 _passthrough_constraints_val = read_root_config("buck2", "passthrough_constraints", "")