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/BUILD.bazel

@@ -11,7 +11,6 @@ ts_library(
         ],
     ),
     module_name = "@angular/compiler",
-    # module_root = "index.d.ts",
 )
 
 ng_package(
@@ -22,9 +21,6 @@ ng_package(
     ],
     entry_point = "packages/compiler/compiler.js",
     include_devmode_srcs = True,
-    secondary_entry_points = [
-        "testing",
-    ],
     deps = [
         ":compiler",
         "//packages/compiler/testing",