Revert auto-channel detection that produced irrecoverable edge-cases

In more advanced usage scenarios, the guarantees of this code path
are never strong enough and the error produced is non-recoverable.

Sorry to lovers of "automagic".

examples/exporters/flake.nix

@@ -16,6 +16,7 @@
       inherit self inputs;
 
       # Channel specific overlays. Overlays `coreutils` from `unstable` channel.
+      channels.unstable = {};
       channels.nixpkgs.overlaysBuilder = channels: [
         (final: prev: { inherit (channels.unstable) ranger; })
       ];

examples/home-manager+nur+neovim/flake.nix

@@ -26,6 +26,7 @@
 
 
       channelsConfig.allowUnfree = true;
+      channels.nixpkgs = {};
 
       sharedOverlays = [
         nur.overlay

examples/minimal-multichannel/flake.nix

@@ -16,6 +16,8 @@
       # Channels are automatically generated from nixpkgs inputs
       # e.g the inputs which contain `legacyPackages` attribute are used.
       channelsConfig.allowUnfree = true;
+      channels.nixpkgs = {};
+      channels.unstable = {};
 
 
       # Modules shared between all hosts

lib/mkFlake.nix

@@ -105,6 +105,16 @@ let
   ];
 
   getChannels = system: self.pkgs.${system};
+  ensureChannelsWitsInputs = mapAttrs (n: v:
+    if (!v ? input) then
+      v // {
+        input = inputs.${n} or (
+        throw ''
+          No input is inferable by name from flake inputs for channel "${n}"
+        '');
+      }
+    else v
+  ) channels;
   getNixpkgs = host: (getChannels host.system).${host.channelName};
 
   configurationBuilder = reverseDomainName: host': (
@@ -121,7 +131,7 @@ let
       selectedNixpkgs = getNixpkgs host;
       host = evalHostArgs (mergeAny hostDefaults host');
       patchedChannel = selectedNixpkgs.path;
-      channels = getChannels host.system;
+      channels' = getChannels host.system;
 
       specialArgs = host.specialArgs // { channel = selectedNixpkgs; };
 
@@ -133,10 +143,10 @@ let
       nixosSpecialArgs =
         let
           f = channelName:
-            { "${channelName}ModulesPath" = toString (channels.${channelName}.input + "/nixos/modules"); };
+            { "${channelName}ModulesPath" = toString (channels'.${channelName}.input + "/nixos/modules"); };
         in
         # Add `<channelName>ModulesPath`s
-        (foldl' (lhs: rhs: lhs // rhs) { } (map f (attrNames channels)))
+        (foldl' (lhs: rhs: lhs // rhs) { } (map f (attrNames channels')))
         # Override `modulesPath` because otherwise imports from there will not use patched nixpkgs
         // { modulesPath = toString (patchedChannel + "/nixos/modules"); };
 
@@ -220,14 +230,6 @@ mergeAny otherArguments (
   eachSystem supportedSystems
     (system:
       let
-        filterAttrs = pred: set:
-          listToAttrs (concatMap (name: let value = set.${name}; in if pred name value then [ ({ inherit name value; }) ] else [ ]) (attrNames set));
-
-        # Little hack, we make sure that `legacyPackages` contains `nix` to make sure that we are dealing with nixpkgs.
-        # For some odd reason `devshell` contains `legacyPackages` out put as well
-        channelFlakes = filterAttrs (_: value: value ? legacyPackages && value.legacyPackages.x86_64-linux ? nix) inputs;
-        channelsFromFlakes = mapAttrs (name: input: { inherit input; }) channelFlakes;
-
         importChannel = name: value: (import (patchChannel system value.input (value.patches or [ ])) {
           inherit system;
           overlays = [
@@ -239,7 +241,7 @@ mergeAny otherArguments (
           config = channelsConfig // (value.config or { });
         }) // { inherit name; inherit (value) input; };
 
-        pkgs = mapAttrs importChannel (mergeAny channelsFromFlakes channels);
+        pkgs = mapAttrs importChannel ensureChannelsWitsInputs;
 
 
         deprecatedBuilders = channels: { }