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

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

pmb/sideload/__init__.py

@@ -80,13 +80,13 @@ def ssh_install_apks(args: PmbArgs, user, host, port, paths: list) -> None:
     pmb.helpers.run.user(command, output="interactive")
 
     logging.info(f"Installing packages at {user}@{host}")
-    add_cmd = [
+    add_cmd_list = [
         "apk",
         "--wait",
         "30",
         "add",
     ] + remote_paths
-    add_cmd = pmb.helpers.run_core.flat_cmd([add_cmd])
+    add_cmd = pmb.helpers.run_core.flat_cmd([add_cmd_list])
     clean_cmd = pmb.helpers.run_core.flat_cmd([["rm"] + remote_paths])
     add_cmd_complete = shlex.quote(f"{su_cmd} {add_cmd} rc=$?; {clean_cmd} exit $rc")
     # Run apk command in a subshell in case the foreign device has a non-POSIX shell.