home-manager: refactor & improve inconsistencies

[nazarewk]

basically quite large refactor of home-manager module

supersedes #208

changes

• switch most of shell to fully-fledged script files with their own dependencies:
  • proper syntax highlighting in IDEs
  • automated shellchecks during builds
  • significantly smaller and easier to identify home.activation snippets
  • most operations are copy-pasteable oneliners for easier debugging
• normalizes a lot of repeated code (getHomeDirCfg @ lib.nix)
• both files and directories are coerced to submodule types (instead of handling both strings and objects throughout the code)
• added method="external" for both files and directories so one can handle mountpoints management themselves (eg: in a NixOS module for root user, which doesn't have dedicated user instance of systemd), see my code

fixes

• fixes home-manager: mounts are not ordered by mountpoint #207: properly orders mountpoint creation based on directory structure (mounts parent dirs before children dirs)
• fixes bindMount logic flaw: There is already an active mount at or below 'folder'! #198 : adds impermanence-hm-mount-info:
  • gathers mountpoint information explicitly from findmnt as opposed to opportunistic mount parsing
  • identifies "dead" mountpoints (when bindfs process dies, but the path is not unmounted properly)
  • exposes information through print-eval mechanism
• fixes looking for fstype bindfs instead of fuse
• fixes killing bindfs processes when restarting home-manager-<USER>.service (might be a mix of linger = true on NixOS) by putting it into background.slice instead
• introduces proper interoperation between home.activation scripts and systemd.user.services, shadowing mounts happened A LOT during my testing of the new code and services depending on those mountpoints

backward incompatibilities

• switched systemd service naming schemes from bindMount-<shell-escaped then sanitized source dir> to bindMount--<sanitized mountpoint> without further escaping
• files & directories are automatically converted to objects

[nazarewk]

FYI: using home-manager mounts seems very slow, I have switched to external mounts through NixOS for A LOT better performance