avoid include of pure python wheels in packages

[hgy59]

Discussion started in #4475 (comment)

To reduce the spk package size, only cross compiled wheels should be included in the package (wheelhouse).
Pure python packages should always be downloaded at installation time.

For an unknown reason all *.whl files are renamed to *-none-any.whl" for the package at post_wheel_target in spksrc.wheel.mk:

post_wheel_target: $(WHEEL_TARGET)
  @if [ -d "$(WORK_DIR)/wheelhouse" ] ; then \
    mkdir -p $(STAGING_INSTALL_PREFIX)/share/wheelhouse ; \
      cd $(WORK_DIR)/wheelhouse && \
        for w in *.whl; do \
        cp -f $$w $(STAGING_INSTALL_PREFIX)/share/wheelhouse/`echo $$w | cut -d"-" -f -3`-none-any.whl; \
      done ; \
  fi

An initial solution would copy only wheels that are not "*-none-any.whl" in post_wheel_target (and ommit the renaming).

The real solution should avoid to (download and) cross compile pure python packages at all and not only skip those in post_wheel_target.
It might be a challenge to recognize pure python packages without download.

Side effect:
The installer code of all packages that install wheels must be adjusted to include download form pypi (ommit --no-index) and must use the requirements file and not *.whl alone.

[hgy59]

implemented with #4986 et al.