Properly support building pure Python packages

[vyasr]

Most RAPIDS wheels contain extension modules. However, after #33 we will have a number of pure C++ wheels that contain no Python code at all. We also have a handful of pure Python packages, namely dask_cudf and the wheels in the cugraph repo aside from cugraph and pylibcugraph. Those packages are handled in a somewhat specialized manner in the wheels workflows in order to produce pure Python wheels, but we do not handle this correctly for conda packages, where we still produce a package per minor version of Python. We should address this issue more holistically.

There are two parts to this request:

1. Updating our shared workflows to support building pure wheels. The most important thing to do here is to create a new workflows based on wheels-build and conda-python-build that only use a single version. We already do this manually in a few places (especially in the new jobs added in addressing Support dynamic linking between RAPIDS wheels #33), so the simplest solution I see here is creating workflows that wrap those preexisting workflows but pass in a matrix filter containing a max_by(py_ver). The other thing that we may want to do here is forward along any other information specific to pure wheel builds. One example is the need to specify the RAPIDS_PY_WHEEL_PURE variable for various gha-tools to work correctly. We could set that appropriately to the environment of all jobs using this shared workflow.
2. Updating conda recipes to properly produce packages without a Python ABI dependence. This will require that we remove the Python component of the build string, that we specify the packages as noarch:python, and that we ensure that the Python dependency becomes a >=min_version instead of pinning to a specific version (this should automatically be handled if the package is built as noarch:python).