pmb.helpers.pmaports.get_repo: "systemd" or None (MR 2405)

This function is used by "pmbootstrap repo_missing", which is only used
by bpo. In order to support building the split repository in bpo, this
patch is required.

Return "systemd" for the packages that are in extra-repos/systemd, and
None for all other packages (indicating that they don't get split into a
separate repository).

Checking the parent-parent dir of the APKBUILD is a bit fragile
(assuming we don't have subdirs in the systemd repository), but that
assumption was made with the previous implementation as well (would only
return "systemd" in that case). I don't see a better solution with
reasonable effort right now, given that we also support multiple
pmaports dirs. We can improve this in the future, meanwhile let's just
not use subdirs in the systemd repository.

pmb/helpers/pmaports.py

@@ -275,12 +275,16 @@ def get_repo(pkgname) -> str | None:
     """Get the repository folder of an aport.
 
     :pkgname: package name
-    :returns: a string like "main", "device", "cross", ...
-                  or None when the aport could not be found
+    :returns: * None if pkgname is not in extra-repos/
+              * "systemd" if the pkgname is in extra-repos/systemd/
     """
     aport: Path
     aport = find(pkgname)
-    return aport.parent.name
+
+    if aport.parent.parent.name == "extra-repos":
+        return aport.parent.name
+
+    return None
 
 
 def check_arches(arches, arch: Arch):