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/core/context.py

@@ -39,7 +39,7 @@ class Context:
     config: Config
 
     def __init__(self, config: Config):
-        self.log = config.work / "log.txt"
+        self.log = config.cache / "log.txt"
         self.config = config