Before the PR the `config_setting` names where following an internal
logic and those names would be leaking into the error messages when no
match is found. I thought that the names thus should be improved and
maybe made more similar to the `whl` filename parts that they are
derived from.

As part of this change I have also added more docs and added them to
sphinxdocs in the hopes that this documentation helps maintainers and
users looking at error messages alike.

Summary:
* Make names more similar to the whl filenames.
* Instead of having `osx_<cpu>_universal2` config settings for each
  `cpu` value, have a single `osx_universal2` config setting.
* Stop creating redundant/unused config settings
* Refactor the `_dist_config_setting` code to be simpler and create
  fewer targets by using a clever trick for the `whl` config setting
  flag value usage.

The stats:
```
$ bazel query //tests/pypi/config_settings/... | rg ":(|_)is" | wc -l
2223
$ bazel query @dev_pip//_config/... | wc -l
1982

$ bazel query //tests/pypi/config_settings/... | rg ":(|_)is" | wc -l
1780
$ bazel query @dev_pip//_config/... | wc -l
1066
```

Work towards bazel-contrib#260