build: expose flatModuleOutFile option on ng_module (#22814)

This lets projects like Material change ng_package "bundle index" files to non-conflicting paths

Currently packages like @angular/core ship with the generated metadata
in a path like 'core.js' which overwrites one of the inputs.

Angular material puts the generated file in a path like 'index.js'

Either way these files generated by ng_module rules have the potential
to collide with inputs given by the user, which results in an error.

Instead, give users the freedom to choose a different non-conflicting name.

Also this refactors the ng_package rule, removing the redundant
secondary_entry_points attribute.

Instead, we assume that any ng_module in the deps with a module_name
attribute is a secondary entry point.

PR Close #22814

packages/compiler-cli/integrationtest/bazel/ng_module/BUILD.bazel

@@ -4,6 +4,7 @@ ng_module(
     name = "test_module",
     srcs = glob(["*.ts"]),
     entry_point = "index.ts",
+    flat_module_out_file = "flat_module_filename",
     module_name = "some_npm_module",
     deps = ["//packages/core"],
 )