Better docs and typing for `_setup_pkg_and_run` (#46709)

There was a bit of mystery surrounding the arguments for `_setup_pkg_and_run`. It passes
two file descriptors for handling the `gmake`'s job server in child processes, but they are
unsed in the method.

It turns out that there are good reasons to do this -- depending on the multiprocessing
backend, these file descriptors may be closed in the child if they're not passed
directly to it.

- [x] Document all args to `_setup_pkg_and_run`.
- [x] Document all arguments to `_setup_pkg_and_run`.
- [x] Add type hints for `_setup_pkg_and_run`.
- [x] Refactor exception handling in `_setup_pkg_and_run` so it's easier to add type
      hints. `exc_info()` was problematic because it *can* return `None` (just not
      in the context where it's used).  `mypy` was too dumb to notice this.

Signed-off-by: Todd Gamblin <tgamblin@llnl.gov>

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