dartHooks.dartConfigHook: Simplify packageRun function

doc/languages-frameworks/dart.section.md

···
 e.g., for `build_runner`:
 
 ```bash
-packageRun build_runner -- build
+packageRun build_runner build
 ```
 
 Do _not_ use `dart run <package_name>`, as this will attempt to download dependencies with Pub.

pkgs/build-support/dart/build-dart-application/hooks/dart-config-hook.sh

···
     mkdir -p .dart_tool
     cp "$packageConfig" .dart_tool/package_config.json
 
-    # Runs a Dart executable from a package.
+    packagePath() {
+        jq --raw-output --arg name "$1" '.packages.[] | select(.name == $name) .rootUri | sub("file://"; "")' .dart_tool/package_config.json
+    }
+
+    # Runs a Dart executable from a package with a custom path.
     #
     # Usage:
-    # packageRun <package> [executable] [bin_dir]
+    # packageRunCustom <package> [executable] [bin_dir]
     #
     # By default, [bin_dir] is "bin", and [executable] is <package>.
-    # i.e. `packageRun build_runner` is equivalent to `packageRun build_runner build_runner bin`, which runs `bin/build_runner.dart` from the build_runner package.
-    packageRun() {
+    # i.e. `packageRunCustom build_runner` is equivalent to `packageRunCustom build_runner build_runner bin`, which runs `bin/build_runner.dart` from the build_runner package.
+    packageRunCustom() {
         local args=()
         local passthrough=()
 
···
         local path="${args[1]:-$name}"
         local prefix="${args[2]:-bin}"
 
-        local packagePath="$(jq --raw-output --arg name "$name" '.packages.[] | select(.name == $name) .rootUri | sub("file://"; "")' .dart_tool/package_config.json)"
-        dart --packages=.dart_tool/package_config.json "$packagePath/$prefix/$path.dart" "${passthrough[@]}"
+        dart --packages=.dart_tool/package_config.json "$(packagePath "$name")/$prefix/$path.dart" "${passthrough[@]}"
+    }
+
+    # Runs a Dart executable from a package.
+    #
+    # Usage:
+    # packageRun <package> [-e executable] [...]
+    #
+    # To run an executable from an unconventional location, use packageRunCustom.
+    packageRun() {
+        local name="$1"
+        shift
+
+        local executableName="$name"
+        if [ "$1" = "-e" ]; then
+          shift
+          executableName="$1"
+          shift
+        fi
+
+        fileName="$(@yq@ --raw-output --arg name "$executableName" '.executables.[$name] // $name' "$(packagePath "$name")/pubspec.yaml")"
+        packageRunCustom "$name" "$fileName" -- "$@"
     }
 
     echo "Generating the dependency list"

pkgs/build-support/dart/build-dart-application/hooks/default.nix

···
 {
   dartConfigHook = makeSetupHook {
     name = "dart-config-hook";
+    substitutions.yq = "${yq}/bin/yq";
     substitutions.jq = "${jq}/bin/jq";
   } ./dart-config-hook.sh;
   dartBuildHook = makeSetupHook {