feat: Allow defining defaults for hermit.hcl in user config

[lox]

Adds support for user-level configuration defaults in Hermit. Users can now define default settings in their ~/.hermit.hcl file that will be applied to all new Hermit environments.

Example ~/.hermit.hcl:

defaults {
  sources = [
    "https://github.com/lox/private-hermit-packages.git", 
    "https://github.com/cashapp/hermit-packages.git"
  ]
}

Also allows the user config file to be defined with HERMIT_USER_CONFIG or --user-config.

This replaces the more narrowly scoped #434.

[lox]

Possibly controversial, I removed the resolver for user-config and replaced it with basic overrides in init_cmd.go and activate_cmd.go. They were the only places using it and it seemed not worth the cognitive complexity of the resolver mechanism (and it created a chicken/egg problem where I needed to resolve user-config path before I could create the resolver).

I am sure there is some magic to make this work in the resolver, feel free to tell me what I should do there @alecthomas.

[alecthomas]

Glorious! Thanks @lox !