Relocate serde_derive version constraint from serde to serde_core

This disallows using an old version of serde_derive against a new
version of serde_core (with a rename to serde in Cargo.toml).

serde/Cargo.toml

@@ -34,14 +34,6 @@ rustdoc-args = [
     "--extern-html-root-url=std=https://doc.rust-lang.org",
 ]
 
-# This cfg cannot be enabled, but it still forces Cargo to keep serde_derive's
-# version in lockstep with serde's, even if someone depends on the two crates
-# separately with serde's "derive" feature disabled. Every serde_derive release
-# is compatible with exactly one serde release because the generated code
-# involves nonpublic APIs which are not bound by semver.
-[target.'cfg(any())'.dependencies]
-serde_derive = { version = "=1.0.219", path = "../serde_derive" }
-
 
 ### FEATURES #################################################################
 

serde_core/Cargo.toml

@@ -29,6 +29,14 @@ rustdoc-args = [
     "--extern-html-root-url=std=https://doc.rust-lang.org",
 ]
 
+# This cfg cannot be enabled, but it still forces Cargo to keep serde_derive's
+# version in lockstep with serde's, even if someone depends on the two crates
+# separately with serde's "derive" feature disabled. Every serde_derive release
+# is compatible with exactly one serde release because the generated code
+# involves nonpublic APIs which are not bound by semver.
+[target.'cfg(any())'.dependencies]
+serde_derive = { version = "=1.0.219", path = "../serde_derive" }
+
 
 ### FEATURES #################################################################