.gitlab: Move cp-plug-in-subfolder.py to build/meson

It is not a CI-specific file.

libgimp/tests/meson.build

@@ -52,7 +52,7 @@ foreach test_name : tests
   custom_target(basename + '.dummy',
                 input: [ c_test_exe ],
                 output: [ basename + '.dummy' ],
-                command: [ python, meson.project_source_root() / '.gitlab/cp-plug-in-subfolder.py',
+                command: [ python, meson.project_source_root() / 'build/meson/cp-plug-in-subfolder.py',
                            c_test_exe, meson.current_build_dir() / 'c-tests' / basename,
                            '@OUTPUT@' ],
                 build_by_default: true,

plug-ins/common/meson.build

@@ -245,7 +245,7 @@ foreach plugin : common_plugins_list
   custom_target('test-' + plugin_name,
                 input: [ plugin_exe ],
                 output: [ plugin_name + '.dummy' ],
-                command: [ python, meson.project_source_root() / '.gitlab/cp-plug-in-subfolder.py',
+                command: [ python, meson.project_source_root() / 'build/meson/cp-plug-in-subfolder.py',
                            plugin_exe, meson.current_build_dir() / 'test-plug-ins' / plugin_name,
                            '@OUTPUT@' ],
                 build_by_default: true)

plug-ins/python/meson.build

@@ -32,7 +32,7 @@ foreach plugin : plugins
     # from the build directory without installing it.
     # To be run even if we don't install Python plug-ins so that we can
     # still run them for in-build scripts.
-    run_command(python, meson.project_source_root() / '.gitlab/cp-plug-in-subfolder.py',
+    run_command(python, meson.project_source_root() / 'build/meson/cp-plug-in-subfolder.py',
                 meson.current_source_dir() / src, meson.current_build_dir() / name, check: true)
   endforeach
 endforeach