refactor(@angular-devkit/core): use package exports field for secondary entries

The secondary entry point (`node`) within the `@angular-devkit/core` package is
now defined by an exports entry in the main package.json file for the package. This
removes the need for the additional nested package.json file in a subdirectory.
The exports field also contains backward compatible entries to allow for continued
deep imports into the package. These compatibility entries will, however, be removed
in a future major version. Deep imported files are not considered part of the public
API of the package.

Author: clydin

Diff for: goldens/public-api/angular_devkit/core/node/testing/index.md

@@ -0,0 +1,29 @@
+## API Report File for "@angular-devkit/core_node_testing"
+
+> Do not edit this file. It is a report generated by [API Extractor](https://api-extractor.com/).
+
+```ts
+
+/// <reference types="node" />
+
+import * as fs from 'fs';
+import { Observable } from 'rxjs';
+
+// @public
+export class TempScopedNodeJsSyncHost extends virtualFs.ScopedHost<fs.Stats> {
+    constructor();
+    // (undocumented)
+    get files(): Path[];
+    // (undocumented)
+    get root(): Path;
+    // (undocumented)
+    protected _root: Path;
+    // (undocumented)
+    get sync(): virtualFs.SyncDelegateHost<fs.Stats>;
+    // (undocumented)
+    protected _sync?: virtualFs.SyncDelegateHost<fs.Stats>;
+}
+
+// (No @packageDocumentation comment for this package)
+
+```

Diff for: packages/angular_devkit/core/BUILD.bazel

@@ -88,7 +88,6 @@ pkg_npm(
         ":core",
         ":license",
         "//packages/angular_devkit/core/node",
-        "//packages/angular_devkit/core/node:package.json",
         "//packages/angular_devkit/core/node/testing",
     ],
 )

Diff for: packages/angular_devkit/core/node/BUILD.bazel

@@ -20,7 +20,6 @@ ts_library(
             "**/*_spec.ts",
         ],
     ),
-    data = ["package.json"],
     module_name = "@angular-devkit/core/node",
     module_root = "index.d.ts",
     deps = [

Diff for: packages/angular_devkit/core/node/host_spec.ts

@@ -7,12 +7,11 @@
  */
 
 /* eslint-disable @typescript-eslint/no-non-null-assertion */
-/* eslint-disable import/no-extraneous-dependencies */
-import { normalize, virtualFs } from '@angular-devkit/core';
-import { NodeJsAsyncHost, NodeJsSyncHost } from '@angular-devkit/core/node';
 import * as fs from 'fs';
 import { tmpdir } from 'os';
 import { join } from 'path';
+import { normalize, virtualFs } from '../src';
+import { NodeJsAsyncHost, NodeJsSyncHost } from './host';
 
 // TODO: replace this with an "it()" macro that's reusable globally.
 let linuxOnlyIt: typeof it = it;

Diff for: packages/angular_devkit/core/node/package.json

@@ -7,6 +7,23 @@
   "keywords": [
     "core"
   ],
+  "exports": {
+    ".": {
+      "types": "./src/index.d.ts",
+      "default": "./src/index.js"
+    },
+    "./node": {
+      "types": "./node/index.d.ts",
+      "default": "./node/index.js"
+    },
+    "./node/testing": {
+      "types": "./node/testing/index.d.ts",
+      "default": "./node/testing/index.js"
+    },
+    "./package.json": "./package.json",
+    "./*": "./*.js",
+    "./*.js": "./*.js"
+  },
   "dependencies": {
     "ajv-formats": "2.1.1",
     "ajv": "8.12.0",