feat(ivy): able to compile @angular/core with ngtsc (#24677)

@angular/core is unique in that it defines the Angular decorators
(@Component, @Directive, etc). Ordinarily ngtsc looks for imports
from @angular/core in order to identify these decorators. Clearly
within core itself, this strategy doesn't work.

Instead, a special constant ITS_JUST_ANGULAR is declared within a
known file in @angular/core. If ngtsc sees this constant it knows
core is being compiled and can ignore the imports when evaluating
decorators.

Additionally, when compiling decorators ngtsc will often write an
import to @angular/core for needed symbols. However @angular/core
cannot import itself. This change creates a module within core to
export all the symbols needed to compile it and adds intelligence
within ngtsc to write relative imports to that module, instead of
absolute imports to @angular/core.

PR Close #24677

packages/core/src/r3_symbols.ts

@@ -0,0 +1,32 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+/*
+ * This file exists to support compilation of @angular/core in Ivy mode.
+ *
+ * When the Angular compiler processes a compilation unit, it normally writes imports to
+ * @angular/core. When compiling the core package itself this strategy isn't usable. Instead, the
+ * compiler writes imports to this file.
+ *
+ * Only a subset of such imports are supported - core is not allowed to declare components or pipes.
+ * A check in ngtsc's translator.ts validates this condition.
+ *
+ * The below symbols are used for @Injectable and @NgModule compilation.
+ */
+
+export {InjectableDef, InjectorDef, defineInjectable, defineInjector} from './di/defs';
+export {inject} from './di/injector';
+export {NgModuleDef} from './metadata/ng_module';
+export {defineNgModule as ɵdefineNgModule} from './render3/definition';
+
+
+/**
+ * The existence of this constant (in this particular file) informs the Angular compiler that the
+ * current program is actually @angular/core, which needs to be compiled specially.
+ */
+export const ITS_JUST_ANGULAR = true;