nixos/modules: Add declarationsWithLocations

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

$ nix repl .
nix-repl> :p nixosConfigurations.micro.options.environment.systemPackages.declarationsWithLocations
[ { 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#65024

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

@@ -78,6 +78,9 @@ let
                   # there's _module.args. If specialArgs.modulesPath is defined it will be
                   # used as the base path for disabledModules.
                   specialArgs ? {}
+                , # Whether to get declaration locations. This has some
+                  # performance cost to evaluation time.
+                  declarationLocations ? false
                 , # `class`:
                   # A nominal type for modules. When set and non-null, this adds a check to
                   # make sure that only compatible modules are imported.
@@ -459,7 +462,7 @@ let
         else config;
     in
     if m ? config || m ? options then
-      let badAttrs = removeAttrs m ["_class" "_file" "key" "disabledModules" "imports" "options" "config" "meta" "freeformType"]; in
+      let badAttrs = removeAttrs m ["_class" "_file" "_loc" "key" "disabledModules" "imports" "options" "config" "meta" "freeformType"]; in
       if badAttrs != {} then
         throw "Module `${key}' has an unsupported attribute `${head (attrNames badAttrs)}'. This is caused by introducing a top-level `config' or `options' attribute. Add configuration attributes immediately on the top level instead, or move all of them (namely: ${toString (attrNames badAttrs)}) into the explicit `config' attribute."
       else
@@ -480,7 +483,7 @@ let
         disabledModules = m.disabledModules or [];
         imports = m.require or [] ++ m.imports or [];
         options = {};
-        config = addFreeformType (removeAttrs m ["_class" "_file" "key" "disabledModules" "require" "imports" "freeformType"]);
+        config = addFreeformType (removeAttrs m ["_class" "_file" "_loc" "key" "disabledModules" "require" "imports" "freeformType"]);
       };
 
   applyModuleArgsIfFunction = key: f: args@{ config, options, lib, ... }:
@@ -537,7 +540,7 @@ 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 +557,11 @@ let
               else
                 mapAttrs
                   (n: option:
-                    [{ inherit (module) _file; options = option; }]
+                    [{ inherit (module) _file; _loc = builtins.unsafeGetAttrPos n subtree; options = option; }]
                   )
                   subtree
               )
-            options);
+            modules);
 
       # The root of any module definition must be an attrset.
       checkedConfigs =
@@ -730,9 +733,10 @@ let
             else res.options;
         in opt.options // res //
           { declarations = res.declarations ++ [opt._file];
+            declarationsWithLocations = res.declarationsWithLocations ++ optional (opt._loc != null) opt._loc;
             options = submodules;
           } // typeSet
-    ) { inherit loc; declarations = []; options = []; } opts;
+    ) { inherit loc; declarations = []; declarationsWithLocations = []; options = []; } opts;
 
   /* Merge all the definitions of an option to produce the final
      config value. */

nixos/lib/eval-config-minimal.nix

@@ -31,13 +31,14 @@ let
     prefix ? [],
     modules ? [],
     specialArgs ? {},
+    declarationLocations ? false,
   }:
   # NOTE: Regular NixOS currently does use this function! Don't break it!
   #       Ideally we don't diverge, unless we learn that we should.
   #       In other words, only the public interface of nixos.evalModules
   #       is experimental.
   lib.evalModules {
-    inherit prefix modules;
+    inherit prefix modules declarationLocations;
     class = "nixos";
     specialArgs = {
       modulesPath = builtins.toString ../modules;

nixos/lib/eval-config.nix

@@ -32,6 +32,7 @@ evalConfigArgs@
 , lib ? import ../../lib
 , extraModules ? let e = builtins.getEnv "NIXOS_EXTRA_MODULE_PATH";
                  in lib.optional (e != "") (import e)
+, declarationLocations ? false
 }:
 
 let pkgs_ = pkgs;
@@ -94,7 +95,7 @@ let
       locatedModules ++ legacyModules;
 
   noUserModules = evalModulesMinimal ({
-    inherit prefix specialArgs;
+    inherit prefix specialArgs declarationLocations;
     modules = baseModules ++ extraModules ++ [ pkgsModule modulesModule ];
   });