App.distro_pkg_ext: Add property

Add .distro_pkg_ext to App, in order to support something along the
lines of

  jw-pkg distro install "https://my-company.com/repos/%{id}/%{codename}/my-base-pkg-latest.%{pkg-ext}"

Signed-off-by: Jan Lindemann <jan@janware.com>

src/python/jw/pkg/App.py

@@ -12,7 +12,7 @@ if TYPE_CHECKING:
     import os, sys, pwd, re
 
 import os, sys, argparse, pwd, re
-from functools import lru_cache
+from functools import lru_cache, cached_property
 from enum import Enum, auto
 
 from .lib.App import App as Base
@@ -374,6 +374,14 @@ class App(Base):
     def distro_cascade(self) -> str:
         return ' '.join(self.os_cascade())
 
+    @cached_property
+    def distro_pkg_ext(self) -> str:
+        for entry in self.os_cascade():
+            ret = entry.replace('pkg-', '')
+            if ret != entry:
+                return ret
+        raise RuntimeError(f'No package extension in found in {self.distro_cascade}')
+
     @property
     def distro_gnu_triplet(self) -> str: