dev-lang/python-exec-conf: New package for python-exec.conf

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.

Closes: https://bugs.gentoo.org/765598
Signed-off-by: Michał Górny <mgorny@gentoo.org>

dev-lang/python-exec-conf/Manifest

@@ -0,0 +1 @@
+DIST python-exec-2.4.6.tar.bz2 87634 BLAKE2B 7b6de8ad0e0603fafd3284e6e3c5247ad83f145ab4db6728914318ae8e6f5aaa3c0247f4e01238fca11519ef72fb1b11436aea7e2b8c988b8717b3f6a2a43c37 SHA512 e05eaf01b83de196a10933636ab6b1a5489a421592df49b8b58eabd0e732de970f902744cd3a06b5ab530a6d69fe6dfa8f270fbb09b9fe3df4ae04d516828050

dev-lang/python-exec-conf/metadata.xml

@@ -0,0 +1,8 @@
+<?xml version='1.0' encoding='UTF-8'?>
+<!DOCTYPE pkgmetadata SYSTEM "http://www.gentoo.org/dtd/metadata.dtd">
+<pkgmetadata>
+	<maintainer type="project">
+		<email>python@gentoo.org</email>
+		<name>Python</name>
+	</maintainer>
+</pkgmetadata>

dev-lang/python-exec-conf/python-exec-conf-2.4.6.ebuild

@@ -0,0 +1,40 @@
+# Copyright 1999-2021 Gentoo Authors
+# Distributed under the terms of the GNU General Public License v2
+
+EAPI=7
+
+inherit python-utils-r1
+
+MY_P=${P/-conf}
+DESCRIPTION="Configuration file for dev-lang/python-exec"
+HOMEPAGE="https://github.com/mgorny/python-exec/"
+SRC_URI="https://github.com/mgorny/python-exec/releases/download/v${PV}/${MY_P}.tar.bz2"
+S=${WORKDIR}/${MY_P}
+
+LICENSE="BSD-2"
+SLOT="2"
+KEYWORDS="~alpha ~amd64 ~arm ~arm64 ~hppa ~ia64 ~m68k ~mips ~ppc ~ppc64 ~riscv ~s390 ~sparc ~x86 ~x64-cygwin ~amd64-linux ~x86-linux ~ppc-macos ~x64-macos ~sparc-solaris ~sparc64-solaris ~x64-solaris ~x86-solaris"
+# Internal Python project hack.  Do not copy it.  Ever.
+IUSE="${_PYTHON_ALL_IMPLS[@]/#/python_targets_}"
+
+RDEPEND="!<dev-lang/python-exec-2.4.6-r4"
+
+src_configure() {
+	:
+}
+
+src_install() {
+	local pyimpls=() i EPYTHON
+	for i in "${_PYTHON_ALL_IMPLS[@]}"; do
+		if use "python_targets_${i}"; then
+			_python_export "${i}" EPYTHON
+			pyimpls+=( "${EPYTHON}" )
+		fi
+	done
+
+	# Prepare and own the template
+	insinto /etc/python-exec
+	newins - python-exec.conf \
+		< <(sed -n -e '/^#/p' config/python-exec.conf.example &&
+			printf '%s\n' "${pyimpls[@]}" | tac)
+}