Add lots of type hints, fix some bugs, and clean up some code

I may have gone a bit overboard. See individual commits.

pmb/chroot/run.py

@@ -5,7 +5,7 @@ from pathlib import Path, PurePath
 import shutil
 import subprocess
 from collections.abc import Sequence
-from typing import cast, overload, Literal
+from typing import overload, Literal
 
 import pmb.config
 import pmb.chroot
@@ -214,7 +214,7 @@ def userm(
     flat_cmd = pmb.helpers.run_core.flat_cmd(cmds, env=env)
     cmd = ["busybox", "su", "pmos", "-c", flat_cmd]
     # Can't figure out why this one fails :(
-    return pmb.chroot.root( # type: ignore[call-overload]
+    return pmb.chroot.root(  # type: ignore[call-overload]
         cmd, chroot, working_dir, output, output_return, check, {}, add_proxy_env_vars=False
     )