Rename rustc_contract to contract

This has now been approved as a language feature and no longer needs
a `rustc_` prefix.

Also change the `contracts` feature to be marked as incomplete and
`contracts_internals` as internal.

compiler/rustc_span/src/symbol.rs

@@ -118,8 +118,8 @@ symbols! {
         MacroRules:         "macro_rules",
         Raw:                "raw",
         Reuse:              "reuse",
-        RustcContractEnsures:  "rustc_contract_ensures",
-        RustcContractRequires: "rustc_contract_requires",
+        ContractEnsures:  "contract_ensures",
+        ContractRequires: "contract_requires",
         Safe:               "safe",
         Union:              "union",
         Yeet:               "yeet",
@@ -682,7 +682,9 @@ symbols! {
         contract_check_ensures,
         contract_check_requires,
         contract_checks,
+        contracts,
         contracts_ensures,
+        contracts_internals,
         contracts_requires,
         convert_identity,
         copy,
@@ -1716,8 +1718,6 @@ symbols! {
         rustc_const_stable,
         rustc_const_stable_indirect,
         rustc_const_unstable,
-        rustc_contracts,
-        rustc_contracts_internals,
         rustc_conversion_suggestion,
         rustc_deallocator,
         rustc_def_path,