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:
- https://github.com/NixOS/nixpkgs/pull/65024

  This one does it for all *definitions* rather than declarations, and
  it was not followed through with due to performance worries.
- https://github.com/NixOS/nixpkgs/pull/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.

1 files changed, 19 insertions, 1 deletions

diff --git a/lib/tests/modules.sh b/lib/tests/modules.sh
index b933a24a57a1..08535b189ee8 100755
--- a/lib/tests/modules.sh
+++ b/lib/tests/modules.sh
@@ -39,7 +39,7 @@ reportFailure() {
 checkConfigOutput() {
     local outputContains=$1
     shift
-    if evalConfig "$@" 2>/dev/null | grep --silent "$outputContains" ; then
+    if evalConfig "$@" 2>/dev/null | grep -E --silent "$outputContains" ; then
         ((++pass))
     else
         echo 2>&1 "error: Expected result matching '$outputContains', while evaluating"
@@ -439,6 +439,24 @@ checkConfigOutput '^"The option `a\.b. defined in `.*/doRename-warnings\.nix. ha
 checkConfigOutput '^"pear"$' config.once.raw ./merge-module-with-key.nix
 checkConfigOutput '^"pear\\npear"$' config.twice.raw ./merge-module-with-key.nix
 
+# Declaration positions
+# Line should be present for direct options
+checkConfigOutput '^10$' options.imported.line10.declarationPositions.0.line ./declaration-positions.nix
+checkConfigOutput '/declaration-positions.nix"$' options.imported.line10.declarationPositions.0.file ./declaration-positions.nix
+# Generated options may not have line numbers but they will at least get the
+# right file
+checkConfigOutput '/declaration-positions.nix"$' options.generated.line18.declarationPositions.0.file ./declaration-positions.nix
+checkConfigOutput '^null$' options.generated.line18.declarationPositions.0.line ./declaration-positions.nix
+# Submodules don't break it
+checkConfigOutput '^39$' config.submoduleLine34.submodDeclLine39.0.line ./declaration-positions.nix
+checkConfigOutput '/declaration-positions.nix"$' config.submoduleLine34.submodDeclLine39.0.file ./declaration-positions.nix
+# New options under freeform submodules get collected into the parent submodule
+# (consistent with .declarations behaviour, but weird; notably appears in system.build)
+checkConfigOutput '^34|23$' options.submoduleLine34.declarationPositions.0.line ./declaration-positions.nix
+checkConfigOutput '^34|23$' options.submoduleLine34.declarationPositions.1.line ./declaration-positions.nix
+# nested options work
+checkConfigOutput '^30$' options.nested.nestedLine30.declarationPositions.0.line ./declaration-positions.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass