i18n: Fix source root in Gettext targets for subprojects

Gettext should search for input files relative to the (sub)project
source root, not the global source root.

This change exposes a root_subdir member in ModuleState.

mesonbuild/modules/__init__.py

@@ -49,6 +49,7 @@ class ModuleState:
                                     interpreter.environment.get_build_dir())
         self.subproject = interpreter.subproject
         self.subdir = interpreter.subdir
+        self.root_subdir = interpreter.root_subdir
         self.current_lineno = interpreter.current_lineno
         self.environment = interpreter.environment
         self.project_name = interpreter.build.project_name

mesonbuild/modules/i18n.py

@@ -256,10 +256,13 @@ class I18nModule(ExtensionModule):
 
         extra_arg = '--extra-args=' + '@@'.join(extra_args) if extra_args else None
 
+        source_root = path.join(state.source_root, state.root_subdir)
+        subdir = path.relpath(state.subdir, start=state.root_subdir) if state.subdir else None
+
         potargs = state.environment.get_build_command() + ['--internal', 'gettext', 'pot', pkg_arg]
-        potargs.append(f'--source-root={state.source_root}')
-        if state.subdir:
-            potargs.append(f'--subdir={state.subdir}')
+        potargs.append(f'--source-root={source_root}')
+        if subdir:
+            potargs.append(f'--subdir={subdir}')
         if datadirs:
             potargs.append(datadirs)
         if extra_arg:
@@ -302,9 +305,9 @@ class I18nModule(ExtensionModule):
         targets.append(allgmotarget)
 
         updatepoargs = state.environment.get_build_command() + ['--internal', 'gettext', 'update_po', pkg_arg]
-        updatepoargs.append(f'--source-root={state.source_root}')
-        if state.subdir:
-            updatepoargs.append(f'--subdir={state.subdir}')
+        updatepoargs.append(f'--source-root={source_root}')
+        if subdir:
+            updatepoargs.append(f'--subdir={subdir}')
         if lang_arg:
             updatepoargs.append(lang_arg)
         if datadirs: