refactor(migrations): ensure CommonJS migrations can be accessed (#43657)

The `@angular/core` migrations are currently published as CommonJS and also not bundled. Schematics, of which migrations are a type, are currently required to be CommonJS modules due to the Schematics Runtime not yet supporting ESM-based schematics. To ensure that the migration files are loaded as CommonJS modules, a nested `package.json` file has been introduced within the `schematics` directory of the package to set the `type` field for all containing JavaScript files as CommonJS. Since the migrations are also not currently bundled and the `devmode` output used to build the migrations will replace all relative imports with fully-qualified module specifiers, an additional `exports` entry must be added to allow the fully-qualified module specifiers to correctly resolve. If `devmode` did not change the relative imports the additional entry would not be needed. Bundling would also remedy the situation and is the long-term path, especially once the migrations are converted to ESM. Additional followup changes should investigate bundling each migration. The additional `exports` entry should be removed if either `devmode` behavior is changed or the migrations are bundled.

PR Close #43657

packages/bazel/test/ng_package/core_package.spec.ts

@@ -72,6 +72,9 @@ describe('@angular/core ng_package', () => {
               node: './fesm2015/testing.mjs',
               default: './fesm2020/testing.mjs',
             },
+            './schematics/*': {
+              'default': './schematics/*.js',
+            },
           }
         }));
       });

packages/core/package.json

@@ -7,6 +7,11 @@
   "engines": {
     "node": "^12.14.1 || >=14.0.0"
   },
+  "exports": {
+    "./schematics/*": {
+      "default": "./schematics/*.js"
+    }
+  },
   "dependencies": {
     "tslib": "^2.3.0"
   },

packages/core/schematics/BUILD.bazel

@@ -7,7 +7,10 @@ exports_files([
 
 pkg_npm(
     name = "npm_package",
-    srcs = ["migrations.json"],
+    srcs = [
+        "migrations.json",
+        "package.json",
+    ],
     visibility = ["//packages/core:__pkg__"],
     deps = [
         "//packages/core/schematics/migrations/abstract-control-parent",

packages/core/schematics/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "commonjs"
+}