lib/types: Allow types to emit a deprecation warning

Previously the only way to deprecate a type was using

  theType = lib.warn "deprecated" (mkOptionType ...)

This caused the warning to be emitted when the type was evaluated, but
the error didn't include which option actually used that type.

With this commit, types can specify a deprecationMessage, which when
non-null, is printed along with the option that uses the type

lib/modules.nix

@@ -457,7 +457,11 @@ rec {
       # yield a value computed from the definitions
       value = if opt ? apply then opt.apply res.mergedValue else res.mergedValue;
 
-    in opt //
+      warnDeprecation =
+        if opt.type.deprecationMessage == null then id
+        else warn "The type `types.${opt.type.name}' of option `${showOption loc}' defined in ${showFiles opt.declarations} is deprecated. ${opt.type.deprecationMessage}";
+
+    in warnDeprecation opt //
       { value = builtins.addErrorContext "while evaluating the option `${showOption loc}':" value;
         inherit (res.defsFinal') highestPrio;
         definitions = map (def: def.value) res.defsFinal;