options: allow setting subproject options in subproject() call

Something like

   subproject('sub', default_options: ['sub2:from_subp=true'])

will cause an assertion failure due to "key.subproject is None"
obviously being false.  Just support this, since it's easy to do so.

(cherry picked from commit 8e2e5c5)

Author: bonzini

mesonbuild/options.py

@@ -1418,15 +1418,18 @@ def initialize_from_subproject_call(self,
         for o in itertools.chain(project_default_options, spcall_default_options):
             keystr, valstr = o.split('=', 1)
             key = OptionKey.from_string(keystr)
-            assert key.subproject is None
-            key = key.evolve(subproject=subproject)
+            if key.subproject is None:
+                key = key.evolve(subproject=subproject)
+            elif key.subproject == subproject:
+                without_subp = key.evolve(subproject=None)
+                raise MesonException(f'subproject name not needed in default_options; use "{without_subp}" instead of "{key}"')
             # If the key points to a project option, set the value from that.
             # Otherwise set an augment.
             if key in self.project_options:
                 self.set_option(key, valstr, is_first_invocation)
             else:
                 self.pending_options.pop(key, None)
-                aug_str = f'{subproject}:{keystr}'
+                aug_str = str(key)
                 self.augments[aug_str] = valstr
         # Check for pending options
         assert isinstance(cmd_line_options, dict)