modules: Fix paths to (sub)project source/build directories

The subproject directory name (i.e. 'subprojects') was being added
to the path even for the main project.

mesonbuild/modules/gnome.py

@@ -1185,8 +1185,8 @@ class GnomeModule(ExtensionModule):
         scan_command += list(scan_external_ldflags)
 
         if self._gir_has_option('--sources-top-dirs'):
-            scan_command += ['--sources-top-dirs', os.path.join(state.environment.get_source_dir(), self.interpreter.subproject_dir, state.subproject)]
-            scan_command += ['--sources-top-dirs', os.path.join(state.environment.get_build_dir(), self.interpreter.subproject_dir, state.subproject)]
+            scan_command += ['--sources-top-dirs', os.path.join(state.environment.get_source_dir(), state.root_subdir)]
+            scan_command += ['--sources-top-dirs', os.path.join(state.environment.get_build_dir(), state.root_subdir)]
 
         if '--warn-error' in scan_command:
             FeatureDeprecated.single_use('gnome.generate_gir argument --warn-error', '0.55.0',

mesonbuild/modules/hotdoc.py

@@ -177,9 +177,8 @@ class HotdocTargetBuilder:
 
         value, _ = self.get_value([list, str], 'gi_c_source_roots', default=[], force_list=True)
         value.extend([
-            os.path.join(self.state.environment.get_source_dir(),
-                         self.interpreter.subproject_dir, self.state.subproject),
-            os.path.join(self.state.environment.get_build_dir(), self.interpreter.subproject_dir, self.state.subproject)
+            os.path.join(self.sourcedir, self.state.root_subdir),
+            os.path.join(self.builddir, self.state.root_subdir)
         ])
 
         self.cmd += ['--gi-c-source-roots'] + value