Split python-exec.conf file to a separate package, so that we can use
PYTHON_TARGETS to control its default contents. This can be used
to ensure that newer Python implementations are not used by default
unless the user actually enables the relevant target.
Note that we can't reuse PYTHON_TARGETS in dev-lang/python-exec this
way. They are used to ensure that dev-lang/python-exec is rebuilt
with the correct implementation list.
Signed-off-by: Michał Górny <email@example.com>