package/pkg-python: migrate flit to new bootstrapping sequence

There are a number of flit toolchain dependencies currently in the
process of deprecating distutils based fallbacks.

This will be needed in order to update tomli.

We need to migrate these to use a new bootstrap based build+install
sequence which relies on flit's bootstrap wheel build+install
features to build and install host-python-pypa-build and
host-python-installer which gives us a full pep517 toolchain.

Note that one can run host-python-flit-core commands for building
and installing itself since the package build directory is the cwd.

We need to add a special flit-bootstrap SETUP_TYPE for dependencies
of host-python-pypa-build and host-python-installer which can not
use the normal flit SETUP_TYPE which would cause a circular dependency
issue.

To avoid further special casing for host-python-flit-core and
host-python-installer, we add two additional changes:
- _BASE_INSTALL_CMD is set with ?=, so it can be overridden by the
  package;
- No _DEPENDENCIES are added for host-python-flit-core and
  host-python-installer, to avoid circular dependencies. These two are
  responsible for their own dependencies.

Although flit-bootstrap _BASE_ variables are almost identical to the
flit/pep517 variables, only _BASE_ENV is really identical. To make
things easier to understand, treat flit-bootstrap as a completely
separate SETUP_TYPE. This also allows us to check that it is only used
for host packages.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle <arnout@mind.be>
[Arnout: simplify things by delegating some special casing to the
packages themselves; add check that it's only for host.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-python.mk

@@ -154,6 +154,9 @@ HOST_PKG_PYTHON_PEP517_INSTALL_OPTS = \
 	--scripts=$(HOST_DIR)/bin \
 	--data=$(HOST_DIR)
 
+HOST_PKG_PYTHON_PEP517_BOOTSTRAP_INSTALL_OPTS = \
+	--installdir=$(HOST_DIR)/lib/python$(PYTHON3_VERSION_MAJOR)/site-packages
+
 ################################################################################
 # inner-python-package -- defines how the configuration, compilation
 # and installation of a Python package should be done, implements a
@@ -214,6 +217,14 @@ $(2)_BASE_ENV = $$(HOST_PKG_PYTHON_PEP517_ENV)
 $(2)_BASE_BUILD_CMD = -m build -n -w
 $(2)_BASE_INSTALL_CMD = $(TOPDIR)/support/scripts/pyinstaller.py dist/* $$(HOST_PKG_PYTHON_PEP517_INSTALL_OPTS)
 endif
+else ifeq ($$($(2)_SETUP_TYPE),flit-bootstrap)
+ifeq ($(4),target)
+$$(error flit-bootstrap setup type only supported for host packages)
+else
+$(2)_BASE_ENV = $$(HOST_PKG_PYTHON_PEP517_ENV)
+$(2)_BASE_BUILD_CMD = -m flit_core.wheel
+$(2)_BASE_INSTALL_CMD ?= $(TOPDIR)/support/scripts/pyinstaller.py dist/* $$(HOST_PKG_PYTHON_PEP517_INSTALL_OPTS)
+endif
 else
 $$(error "Invalid $(2)_SETUP_TYPE. Valid options are 'distutils', 'setuptools', 'pep517' or 'flit'.")
 endif
@@ -239,6 +250,10 @@ $(2)_DEPENDENCIES += host-python-pypa-build host-python-installer
 ifeq ($$($(2)_SETUP_TYPE),flit)
 $(2)_DEPENDENCIES += host-python-flit-core
 endif
+else ifeq ($$($(2)_SETUP_TYPE),flit-bootstrap)
+ifeq ($$(filter host-python-flit-core host-python-installer,$(1)),)
+$(2)_DEPENDENCIES += host-python-flit-core host-python-installer
+endif
 endif # SETUP_TYPE
 
 # Python interpreter to use for building the package.