package/python-installer: migrate setup type to flit

This package is moving to flit and will soon be dropping distutils
compatibility support.

We need to use flit-bootstrap as opposed to the normal flit setup
type since host-python-pypa-build depends on host-python-installer.

We need to add the src directory to the PYTHONPATH so that installer
can run from the src directory when installing itself.

We need to explicitly add host-python-flit-core to the dependencies -
only host-python-installer is automatically added to the depenedencies
for flit-bootstrap packages, and this would create a circular dependency
so is explicitly excluded in the infra.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/python-installer/python-installer.mk

@@ -9,6 +9,8 @@ PYTHON_INSTALLER_SOURCE = installer-$(PYTHON_INSTALLER_VERSION).tar.gz
 PYTHON_INSTALLER_SITE = https://files.pythonhosted.org/packages/74/b7/9187323cd732840f1cddd6a9f05961406636b50c799eef37c920b63110c0
 PYTHON_INSTALLER_LICENSE = MIT
 PYTHON_INSTALLER_LICENSE_FILES = LICENSE
-PYTHON_INSTALLER_SETUP_TYPE = distutils
+PYTHON_INSTALLER_SETUP_TYPE = flit-bootstrap
+HOST_PYTHON_INSTALLER_DEPENDENCIES = host-python-flit-core
+HOST_PYTHON_INSTALLER_ENV = PYTHONPATH="$(@D)/src"
 
 $(eval $(host-python-package))