treewide: split chroots from workdir

Introduce a new "cache" subdirectory in the pmbootstrap workdir, all the
cache and config bits go in here, anything that needs to be accessible
from inside a chroot. The whole dir is then bind-mounted into the chroot
as /cache with appropriate symlinks.

This dir is in the config as config.cache.

In addition, all the cache_* and other config dirs are renamed to
be closer to the names of the equivalent dirs in the chroot (e.g.
abuild-config) and to avoid redundant naming since they are now under a
"cache" dir.

Signed-off-by: Casey Connolly <kcxt@postmarketos.org>

pmb/helpers/apk_static.py

@@ -148,7 +148,7 @@ def extract(version: str, apk_path: Path) -> None:
         )
 
     # Move it to the right path
-    target_path = get_context().config.work / "apk.static"
+    target_path = get_context().config.cache / "apk.static"
     shutil.move(temp_path, target_path)