package/pkg-python: use host-python3-setuptools when needed

package/pkg-python: use host-python3-setuptools when needed

When a package uses "setuptools" as its <pkg>_SETUP_TYPE, we currently
add a dependency on host-python-setuptools. This means that:

 (1) When BR2_PACKAGE_PYTHON=y, the default host Python version is
     Python 2.x, and host-python-setuptools is installed for
     host-python.

 (2) When BR2_PACKAGE_PYTHON3=y, the default host Python version is
     Python 3.x, and host-python-setuptools is installed for
     host-python3.

 (3) When no target Python interpreter is selected, the default host
     Python version is Python 2.x, and host-python-setuptools is
     installed for host-python.

Situations (1) and (3) are problematic for host Python packages that
need Python 3.x. Such packages use <pkg>_NEEDS_HOST_PYTHON = python3,
but if they use setuptools as their setup type, they will not find
setuptools installed for host-python3 in situations (1) and (3)
described above.

We currently have a single package that sets <pkg>_NEEDS_HOST_PYTHON =
python3: host-meson. host-meson generally works because if setuptools
is not found, it falls back to distutils, which is part of the
standard Python library. However, if there is a setuptools version
installed system-wide, it may be picked up, but may not necessarily be
the same version as Buildroot setuptools, potentially causing
problems.

This commit makes the necessary change to the python-package
infrastructure to fix this behavior, by identifying the following
cases:

 - When a host Python package says <pkg>_NEEDS_HOST_PYTHON = python3,
   then we know it wants setuptools installed for host-python3, so we
   use host-python3-setuptools.

 - When a host Python package says <pkg>_NEEDS_HOST_PYTHON = python2,
   then we known it wants setuptools installed for host-python, so we
   use host-python-setuptools.

 - When BR2_PACKAGE_PYTHON3=y, and we have a target package, or a host
   package with no NEEDS_HOST_PYTHON option, then we want setuptools
   installed for host-python3, so we use host-python3-setuptools.

 - When BR2_PACKAGE_PYTHON=y or no target interpreter is enabled at
   all, and we have a target package, or a host package with no
   NEEDS_HOST_PYTHON option, then we want setuptools for host-python,
   so we use host-python-setuptools.

To make this happen, we use host-python3-setuptools introduced in a
previous commit, but we also change host-python-setuptools to force
its installation for host-python. The latter is needed if you build
with BR2_PACKAGE_PYTHON3=y but want to install a Python-based package
that has NEEDS_HOST_PYTHON=python2.

There is one single package that needs be adjusted following this:
lirc-tools, because it is not using the python-package
infrastructure. It directly depends on host-python-setuptools, which
no longer works because host-python-setuptools now only installs for
Python 2.x, while lirc-tools Python binding only supports Python
3.x. Switching to host-python3-setuptools solves this problem.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Asaf Kahlon <asafka7@gmail.com>
Reviewed-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>