build: Use fixed gettext domain for non-subproject extension-tool builds

Clarify how the option is supposed to be used by
 - ignoring it for non-subproject builds
 - enforcing that it is set for subproject builds

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1072

subprojects/extensions-tool/meson.build

@@ -9,7 +9,14 @@ gio_req = '>= 2.56.0'
 gnome = import('gnome')
 i18n  = import('i18n')
 
-package_name = get_option('package_name')
+if meson.is_subproject()
+  package_name = get_option('package_name')
+  assert(package_name != '',
+    'package_name must be specified for subproject builds')
+else
+  package_name = meson.project_name()
+endif
+
 package_version = meson.project_version()
 prefix = get_option('prefix')
 

subprojects/extensions-tool/meson_options.txt

@@ -7,6 +7,5 @@ option('man',
 
 option('package_name',
   type: 'string',
-  value: 'gnome-extensions-tool',
-  description: 'The gettext domain name'
+  description: 'The gettext domain name when used as a subproject'
 )