refactor(ivy): make return value of define(Component|Directive|Pipe|Injector|Injectable) private (#23371)

Ivy definition looks something like this:

```
class MyService {
  static ngInjectableDef = defineInjectable({
    …
  });
}
```

Here the argument to `defineInjectable` is well known public contract which needs
to be honored in backward compatible way between versions. The type of the
return value of `defineInjectable` on the other hand is private and can change
shape drastically between versions without effecting backwards compatibility of
libraries publish to NPM. To our users it is effectively an `OpaqueToken`.

By prefixing the type with `ɵ` we are communicating the the outside world that
the value is not public API and is subject to change without backward compatibility.

PR Close #23371

packages/core/src/di/injection_token.ts

@@ -8,7 +8,8 @@
 
 import {Type} from '../type';
 
-import {InjectableDef, defineInjectable} from './defs';
+import {defineInjectable, ɵInjectableDef} from './defs';
+
 
 /**
  * Creates a token that can be used in a DI Provider.
@@ -52,7 +53,7 @@ export class InjectionToken<T> {
   /** @internal */
   readonly ngMetadataName = 'InjectionToken';
 
-  readonly ngInjectableDef: InjectableDef<T>|undefined;
+  readonly ngInjectableDef: ɵInjectableDef<T>|undefined;
 
   constructor(protected _desc: string, options?: {
     providedIn?: Type<any>| 'root' | null,
@@ -72,5 +73,5 @@ export class InjectionToken<T> {
 }
 
 export interface InjectableDefToken<T> extends InjectionToken<T> {
-  ngInjectableDef: InjectableDef<T>;
+  ngInjectableDef: ɵInjectableDef<T>;
 }