postmarketos-base{-systemd}: provide postmarketos-base-init

Why is this needed?

To be able to test the usr-merge downstream we need to stop
depending on alpine-baselayout. For that, we need to stop depending on
alpine-base. This is a first step to create a "provides" solution
downstream between openrc and systemd systems, so that in a follow-up
commit we can recreate alpine-base downstream without the hard-coded
openrc dependency. The virtual instead of a versioned dependency means
that:
* In theory systemd and openrc could be installed in parallel. In
  practice we avoid that with !openrc in the systemd package and
  !systemd in the postmarketos-base-openrc one. There is a lot more
  work required with provides (both upstream and downstream) regarding
  things like elogind and eudev to be able to even have them
  package-wise ready to be installed in parallel
* There must always be one installed. Here we have 3 scenarios:
  * pmbootstrap install where pmbootstrap will decide which one
  * update from existing install. The virtual makes sure that APK
    won't remove the local package because versions changed
  * Manual APK work, where the provider_priority or the user will
    choose

How does this work? Hypothesis:
* openrc upgrades: the postmarketos-base-openrc remains installed,
  moves from being an "install_if", to be there because it fulfills
  the postmarketos-base-init requirements.
* systemd upgrades: postmarketos-base-systemd is still in
  /etc/apk/world, and it fulfills the new dependency in
  postmarketos-base. Nothing happens.
* openrc installs: pmbootstrap will choose one of
  postmarketos-base-openrc and postmarketos-base-systemd... But which?
  The one with the higher version?
* systemd installs: postmarketos-base-systemd is installed before
  anything else is. That means, postmarketos-base-init will already
  be satisfied when postmarketos-base gets installed, and openrc will
  not be installed

Part-of: https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/6475

extra-repos/systemd/postmarketos-base-systemd/APKBUILD

@@ -3,7 +3,7 @@
 # pmbootstrap installs this, if systemd is selected in "pmbootstrap init".
 pkgname=postmarketos-base-systemd
 pkgver=39
-pkgrel=0
+pkgrel=1
 pkgdesc="Meta package for running postmarketOS with systemd"
 url="https://postmarketos.org"
 arch="noarch"
@@ -38,6 +38,8 @@ depends="
 	systemd-timesyncd
 	tzdata
 	"
+provides="postmarketos-base-init"
+provider_priority=100
 replaces="alpine-baselayout postmarketos-base systemd systemd-logind"
 replaces_priority=100
 install="

main/postmarketos-base/APKBUILD

@@ -1,6 +1,6 @@
 pkgname=postmarketos-base
 pkgver=44
-pkgrel=1
+pkgrel=2
 pkgdesc="Meta package for minimal postmarketOS base"
 url="https://postmarketos.org"
 arch="noarch"
@@ -94,6 +94,11 @@ prepare() {
 }
 
 package() {
+	# pmos-base-init provided by a subpackage in this APKBUILD, and it may not
+	# exist when this is evaluated at the top level. It's moved into this
+	# function to work around it.
+	depends="$depends postmarketos-base-init"
+
 	local i
 	for i in $_source440; do
 		install -Dm440 "$srcdir/$(flatpath "$i")" "$pkgdir/$i"
@@ -144,7 +149,8 @@ ssh() {
 }
 
 openrc() {
-	install_if="$pkgname=$pkgver-r$pkgrel openrc"
+	provides="$pkgname-init"
+	provider_priority=10
 	install="$subpkgname.post-install $subpkgname.post-upgrade"
 	replaces="
 		busybox-openrc
@@ -153,6 +159,7 @@ openrc() {
 		zram-init-openrc
 		"
 	depends="
+		!systemd
 		eudev-openrc
 		logbookd
 		logbookd-openrc