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/aportgen/gcc.py

@@ -24,7 +24,7 @@ def generate(pkgname: str) -> None:
         based_on = "main/gcc6 (from postmarketOS)"
     else:
         raise ValueError(f"Invalid prefix '{prefix}', expected gcc, gcc4 or gcc6.")
-    pmb.helpers.run.user(["cp", "-r", upstream, context.config.work / "aportgen"])
+    pmb.helpers.run.user(["cp", "-r", upstream, context.config.cache / "aportgen"])
 
     # Rewrite APKBUILD
     fields = {