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/compiler-cli/src/ngtsc/util/BUILD.bazel

@@ -9,4 +9,7 @@ ts_library(
         "src/**/*.ts",
     ]),
     module_name = "@angular/compiler-cli/src/ngtsc/util",
+    deps = [
+        "//packages:types",
+    ],
 )

packages/compiler-cli/src/ngtsc/util/src/path.ts

@@ -0,0 +1,26 @@
+/**
+ * @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
+ */
+
+import * as path from 'path';
+
+const TS_DTS_EXTENSION = /(\.d)?\.ts$/;
+
+export function relativePathBetween(from: string, to: string): string|null {
+  let relative = path.posix.relative(path.dirname(from), to).replace(TS_DTS_EXTENSION, '');
+
+  if (relative === '') {
+    return null;
+  }
+
+  // path.relative() does not include the leading './'.
+  if (!relative.startsWith('.')) {
+    relative = `./${relative}`;
+  }
+
+  return relative;
+}