move subzero_core to proxy libs (#12742)

We have a dedicated libs folder for proxy related libraries. Let's move
the subzero_core stub there.

Cargo.toml

@@ -46,10 +46,10 @@ members = [
     "libs/proxy/json",
     "libs/proxy/postgres-protocol2",
     "libs/proxy/postgres-types2",
+    "libs/proxy/subzero_core",
     "libs/proxy/tokio-postgres2",
     "endpoint_storage",
     "pgxn/neon/communicator",
-    "proxy/subzero_core",
 ]
 
 [workspace.package]

proxy/Cargo.toml

@@ -110,7 +110,7 @@ zerocopy.workspace = true
 # uncomment this to use the real subzero-core crate
 # subzero-core = { git = "https://github.com/neondatabase/subzero", rev = "396264617e78e8be428682f87469bb25429af88a", features = ["postgresql"], optional = true }
 # this is a stub for the subzero-core crate
-subzero-core = { path = "./subzero_core", features = ["postgresql"], optional = true}
+subzero-core = { path = "../libs/proxy/subzero_core", features = ["postgresql"], optional = true}
 ouroboros = { version = "0.18", optional = true }
 
 # jwt stuff