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

integration/bazel/src/hello-world/BUILD.bazel

@@ -1,6 +1,6 @@
 package(default_visibility = ["//visibility:public"])
 
-load("@angular//:index.bzl", "ng_module")
+load("@angular//:index.bzl", "ng_module", "ng_package")
 load("@io_bazel_rules_sass//sass:sass.bzl", "sass_binary")
 
 sass_binary(
@@ -17,3 +17,9 @@ ng_module(
     # npm distro of angular there is no ts_library rule to propagate the dep.
     deps = ["@rxjs"],
 )
+
+ng_package(
+    name = "npm_package",
+    entry_point = "src/hello-world/index.js",
+    deps = [":hello-world"],
+)

integration/bazel/src/hello-world/index.ts

@@ -0,0 +1,2 @@
+export * from './hello-world.component';
+export * from './hello-world.module';