Remove setting entry point[s] config, only declare outputs

[jbedard]

Today webpack_bundle modifies the base config file entering the entry_point[s], however sometimes the user wants to set them manually within the webpack config such as dynamic entries while outputting a directory.

The various combinations of entry_point[s], output_dir and settings in the user webpack.config have too many conflicting combinations creating a confusing API, unnecessary logic within the rule etc.

Proposed API: the rule name, entry_point[s], output_dir are only used to specify what the webpack config will output to allow bazel to declare outputs. Setting the entries must be done by the user within the webpack config.