This fixes the issue where a sub-module was required to always have
a pip.parse() call configured for the default Python version if they
used any pip.parse() call. Such a requirement puts sub-modules in an
impossible situation: If they don't have the default version, they'll
get an error. If they register the default version, but also register
a specific version, they'll potentially cause an error if a root module
changes the default to match their specific version (becaues two
pip.parse() calls for the same version are made).

The requirement to have the default version registered for a pip hub was
only present to satisfy the `whl_library_alias` repository rule, which
needed a Ptyhon version to map `//conditions:default` to.

To fix, the `whl_library_alias` rule's `default_version` arg is made
optional. When None is passed, the `//condiitions:default` condition
is replaced with a `no_match_error` setting. This prevents the pip hub
from being used with the version-unaware rules, but that makes sense: no
wheels were setup for that version, so it's not like there is something
that can be used anyways.

Fixes bazel-contrib#1320