Kindsys: Replace "Declaration" with "Definition" (#62515)

* s/Declaration/Definition/g

* s/DeclForGen/DefForGen/g

* Rename some local vars

pkg/kindsys/schema_interface.go

@@ -28,10 +28,10 @@ func (s SchemaInterface) Name() string {
 	return s.name
 }
 
-// Interface returns the cue.Value representing the meta-schema that is the
-// contract between core or custom kinds that compose the meta-schema, and the
-// plugin-declared composable kinds that implement the meta-schema.
-func (s SchemaInterface) Interface() cue.Value {
+// Contract returns the cue.Value representing the meta-schema that is the
+// contract between core/custom kinds that consume schemas that are instances
+// of the SchemaInterface contract, and composable kinds that produce such schemas.
+func (s SchemaInterface) Contract() cue.Value {
 	return s.raw.LookupPath(ip)
 }