python-single-r1.eclass: Introduce PYTHON_{SINGLE,MULTI}_USEDEP API

python-single-r1.eclass: Introduce PYTHON_{SINGLE,MULTI}_USEDEP API

Introduce a new API that replaces PYTHON_USEDEP with
PYTHON_SINGLE_USEDEP variable and PYTHON_MULTI_USEDEP placeholder.
The former can be used directly (or via python_gen_cond_dep) when
depending on other python-single-r1 packages, the latter can be used
via python_gen_cond_dep when depending on python-r1 packages.

Long term, the new API will enable us to remove redundant PYTHON_TARGETS
flags from python-single-r1 packages.

Signed-off-by: Michał Górny <mgorny@gentoo.org>