nixos/modules: Add declarationPositions

What it does: line and column level *declaration* position information:

$ nix repl .
nix-repl> :p nixosConfigurations.micro.options.environment.systemPackages.declarationPositions
[ { column = 7; file = "/nix/store/24aj3k7fgqv3ly7qkbf98qvphasrw9nb-source/nixos/modules/config/system-path.nix"; line = 63; } ]

Use cases:
- ctags over NixOS options, as will be presented at NixCon 2023 ;)
- improving the documentation pages to go to the exact line of the
  declarations.

Related work:
- NixOS/nixpkgs#65024

  This one does it for all *definitions* rather than declarations, and
  it was not followed through with due to performance worries.
- NixOS/nixpkgs#208173

  The basis for this change. This change is just a rebase of that one.
  I split it out to add the capability before adding users of it, in
  order to simplify review. However, the ctags script in there is a
  sample user of this feature.

Benchmarks: conducted by evaluating my own reasonably complex NixOS
configuration with the command:
`hyperfine -S none -w 1 -- "nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath"`

```
Benchmark 1: nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath
  Time (mean ± σ):      8.971 s ±  0.254 s    [User: 5.872 s, System: 1.388 s]
  Range (min … max):    8.574 s …  9.327 s    10 runs

Benchmark 1: nix eval .#nixosConfigurations.snowflake.config.system.build.toplevel.outPath
  Time (mean ± σ):      8.766 s ±  0.160 s    [User: 5.873 s, System: 1.346 s]
  Range (min … max):    8.496 s …  9.033 s    10 runs
```

Summary of results: it seems to be in the noise, this does not cause any
visible regression in times.

lib/modules.nix

@@ -537,7 +537,7 @@ let
     mergeModules' prefix modules
       (concatMap (m: map (config: { file = m._file; inherit config; }) (pushDownProperties m.config)) modules);
 
-  mergeModules' = prefix: options: configs:
+  mergeModules' = prefix: modules: configs:
     let
       # an attrset 'name' => list of submodules that declare ‘name’.
       declsByName =
@@ -554,11 +554,11 @@ let
               else
                 mapAttrs
                   (n: option:
-                    [{ inherit (module) _file; options = option; }]
+                    [{ inherit (module) _file; pos = builtins.unsafeGetAttrPos n subtree; options = option; }]
                   )
                   subtree
               )
-            options);
+            modules);
 
       # The root of any module definition must be an attrset.
       checkedConfigs =
@@ -730,9 +730,16 @@ let
             else res.options;
         in opt.options // res //
           { declarations = res.declarations ++ [opt._file];
+            # In the case of modules that are generated dynamically, we won't
+            # have exact declaration lines; fall back to just the file being
+            # evaluated.
+            declarationPositions = res.declarationPositions
+              ++ (if opt.pos != null
+                then [opt.pos]
+                else [{ file = opt._file; line = null; column = null; }]);
             options = submodules;
           } // typeSet
-    ) { inherit loc; declarations = []; options = []; } opts;
+    ) { inherit loc; declarations = []; declarationPositions = []; options = []; } opts;
 
   /* Merge all the definitions of an option to produce the final
      config value. */

lib/tests/modules/declaration-positions.nix

@@ -0,0 +1,49 @@
+{ lib, options, ... }:
+let discardPositions = lib.mapAttrs (k: v: v);
+in
+# unsafeGetAttrPos is unspecified best-effort behavior, so we only want to consider this test on an evaluator that satisfies some basic assumptions about this function.
+assert builtins.unsafeGetAttrPos "a" { a = true; } != null;
+assert builtins.unsafeGetAttrPos "a" (discardPositions { a = true; }) == null;
+{
+  imports = [
+    {
+      options.imported.line10 = lib.mkOption {
+        type = lib.types.int;
+      };
+
+      # Simulates various patterns of generating modules such as
+      # programs.firefox.nativeMessagingHosts.ff2mpv. We don't expect to get
+      # line numbers for these, but we can fall back on knowing the file.
+      options.generated = discardPositions {
+        line18 = lib.mkOption {
+          type = lib.types.int;
+        };
+      };
+
+      options.submoduleLine34.extraOptLine23 = lib.mkOption {
+        default = 1;
+        type = lib.types.int;
+      };
+    }
+  ];
+
+  options.nested.nestedLine30 = lib.mkOption {
+    type = lib.types.int;
+  };
+
+  options.submoduleLine34 = lib.mkOption {
+    default = { };
+    type = lib.types.submoduleWith {
+      modules = [
+        ({ options, ... }: {
+          options.submodDeclLine39 = lib.mkOption { };
+        })
+        { freeformType = with lib.types; lazyAttrsOf (uniq unspecified); }
+      ];
+    };
+  };
+
+  config = {
+    submoduleLine34.submodDeclLine39 = (options.submoduleLine34.type.getSubOptions [ ]).submodDeclLine39.declarationPositions;
+  };
+}