classes/pypi: don't expose PYPI_ARCHIVE_NAME

classes/pypi: don't expose PYPI_ARCHIVE_NAME

This variable is only used when constructing a SRC_URI and some recipes
think that it's the correct value to assign if the PyPi package name
isn't the same as the recipe name, when PYPI_PACKAGE is actually all
that needs to be set.

Also document the variables we expect the recipe to assign if needed,
and where the PyPi URL structure is documented.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>