Small changes to API generation to help creation of virtual pip.

- Underscore some imports like _print_function so we don't have to
  delete them. Deleting them doesn't work well since they get added
  to our __all__ list before being deleted which makes "import *" fail.
- Give a more unique name to some generated target in the api genrule.
  Doing this since I want multiple genrules that generate the same version
  of the API. (a compat_v1 and root_v1 for example).

PiperOrigin-RevId: 221524470