validate the literal directory "subprojects" when checking sandbox violations

We do not want anyone touching this entire directory tree, but due to
the way it was implemented, we only checked if its direct parent was a
subproject violation. This generally worked, unless people tried to add
`subprojects/` as an include directory.

Patch this hole. It now provides the same warning any sandbox violation
does (but is not currently an error, just a "will become an error in the
future").

mesonbuild/interpreter/interpreter.py

@@ -2681,11 +2681,12 @@ Try setting b_lundef to false instead.'''.format(self.coredata.options[OptionKey
             # /opt/vendorsdk/src/file_with_license_restrictions.c
             return
         project_root = Path(srcdir, self.root_subdir)
+        subproject_dir = project_root / self.subproject_dir
         if norm == project_root:
             return
         if project_root not in norm.parents:
             raise InterpreterException(f'Sandbox violation: Tried to grab {inputtype} {norm.name} outside current (sub)project.')
-        if project_root / self.subproject_dir in norm.parents:
+        if subproject_dir == norm or subproject_dir in norm.parents:
             raise InterpreterException(f'Sandbox violation: Tried to grab {inputtype} {norm.name} from a nested subproject.')
 
     @T.overload