deprecate layout=flat and warn people never to use it

It's broken in various circumstances, no one seems to actually use it,
CI doesn't test it, no one is committed to maintaining it, etc. etc.
etc.

Also, projects doing trivially reasonable things, such as generating
"foo/util.py" and "bar/util.py", create clashing output names. This will
never, ever, ever, ever work with layout=flat.

Closes #996
Closes #1521
Closes #1908
Closes #7133
Closes #7135
Closes #7480
Closes #8378

mesonbuild/msetup.py

@@ -267,6 +267,13 @@ class MesonApp:
             # by Interpreter() init, but this is most visible at the end
             if env.coredata.options[mesonlib.OptionKey('backend')].value == 'xcode':
                 mlog.warning('xcode backend is currently unmaintained, patches welcome')
+            if env.coredata.options[mesonlib.OptionKey('layout')].value == 'flat':
+                mlog.warning('-Dlayout=flat is unsupported and probably broken. It was a failed experiment at '
+                             'making Windows build artifacts runnable while uninstalled, due to PATH considerations, '
+                             'but was untested by CI and anyways breaks reasonable use of conflicting targets in different subdirs. '
+                             'Please consider using `meson devenv` instead. See https://github.com/mesonbuild/meson/pull/9243 '
+                             'for details.')
+
         except Exception as e:
             mintro.write_meson_info_file(b, [e])
             if 'cdf' in locals():