lib/get_host: improve abstraction, turn missconfiguration into a warning

Motivation: A warning should encourage consistent usage of inventory.machines setting targetHost inside the machine should be considered a custom override

Changing the warning strings to avoid the term 'nix'/'json' both inventory and nixos machines are nix features

pkgs/clan-cli/clan_lib/machines/machines.py

@@ -178,7 +178,7 @@ class Machine:
 @dataclass(frozen=True)
 class RemoteSource:
     data: Remote
-    source: Literal["inventory", "nix_machine"]
+    source: Literal["inventory", "machine"]
 
 
 @API.register
@@ -191,15 +191,15 @@ def get_host(
     machine = Machine(name=name, flake=flake)
     inv_machine = machine.get_inv_machine()
 
-    source: Literal["inventory", "nix_machine"] = "inventory"
+    source: Literal["inventory", "machine"] = "inventory"
     host_str = inv_machine.get("deploy", {}).get(field)
 
     if host_str is None:
-        machine.debug(
-            f"'{field}' is not set in inventory, falling back to slower Nix config, set it either through the Nix or json interface to improve performance"
+        machine.warn(
+            f"'{field}' is not set in `inventory.machines.${name}.deploy.targetHost` - falling back to _slower_ nixos option: `clan.core.networking.targetHost`"
         )
         host_str = machine.select(f'config.clan.core.networking."{field}"')
-        source = "nix_machine"
+        source = "machine"
 
     if not host_str:
         return None