feat(bazel): add an ng_package rule (#22221)

This produces a directory following the Angular Package layout spec.

Includes integration test coverage by making a minimal ng_package in integration/bazel.
Unit tests verify the content of the @angular/core and @angular/common packages.

This doesn't totally match our current output, but is good enough to unblock some
early adopters.

It re-uses logic from the rollup_bundle rule in rules_nodejs. It should also
eventually have the .pack and .publish secondary targets like npm_package rule.

PR Close #22221

packages/common/BUILD.bazel

@@ -1,6 +1,6 @@
 package(default_visibility = ["//visibility:public"])
 
-load("//tools:defaults.bzl", "ng_module")
+load("//tools:defaults.bzl", "ng_module", "ng_package")
 
 ng_module(
     name = "common",
@@ -16,3 +16,19 @@ ng_module(
         "@rxjs",
     ],
 )
+
+ng_package(
+    name = "npm_package",
+    entry_point = "packages/common/index.js",
+    secondary_entry_points = [
+        "testing",
+        "http",
+        "http/testing",
+    ],
+    deps = [
+        "//packages/common",
+        "//packages/common/http",
+        "//packages/common/http/testing",
+        "//packages/common/testing",
+    ],
+)