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
Alex Eagle
Jeremy Elbourn