[ feature ] Support global (configurational) pack.toml outside the $PACK_DIR

[buzden]

Related to #316 and chez update problems (which is now may be a common case, since chez 10.1 is released).

When for some reason, e.g. because of updating Chez, we need to reinstall pack, it's easy to forget to backup $PACK_DIR/user/pack.toml file before removing the pack dir. It would be handy if configuration would be stored separately, e.g. in $XDG_CONFIG_HOME/pack/pack.toml or in $HOME/.config/pack/pack.toml or somewhere else.

Having this, we even can suggest running reinstallation script when pack is cannot be run due to version incompatibility.

Implementationally, we can even join $PACK_DIR/user/pack.toml and $XDG_CONFIG_HOME/pack/pack.toml when loading configs, and thus code for storing the current selected collection won't be changed, and everything still will work when no external config is present, or when $XDG_CONFIG_HOME is unwritable.

[buzden]

As a variant (maybe, the simplest one?) we can treat proposed pack.toml from $XDG_CONFIG_HOME or $HOME as just another local pack.toml, the very outmost one. It won't change any logic of the global pack.toml file, while, as far as I can see, still solving the original problem.

[stefan-hoeck]

I'm open for simple variants currently. Nothing fancy. And it must be backwards compatible. So I suggest to

• look in $XDG_CONFIG_HOME/pack/pack.toml for the root pack.toml file
• loock in $PADK_DIR/user/pack.toml for settings that might override the ones from $XDG_CONFIG_HOME but are deprecated (a corresponding warning should be issued)