Delay errors in semanal for proper unreachability information

[A5rocks]

Fixes #20408

This is a draft PR because:

• it doesn't work with dmypy
• it's a bit slower:

=== Results ===

master                    2.697s (0.0%) | stdev 0.012s
f18e550f23d30c454a0f6fc2c8b41eb9e03b4f29 2.732s (+1.3%) | stdev 0.018s

[A5rocks]

Idea for a speedup (so I don't forget): only do this for things that can be skipped unreachable (I.e. stmts in a block, toplevel stmts).

Also note that I haven't cleaned this up, which would involve removing eg the assert False special case and adding a test case for the issue this fixes.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

comtypes (https://github.com/enthought/comtypes)
- comtypes/test/test_server.py:326: error: Module "comtypes.gen" has no attribute "TestLib"  [attr-defined]

egglog-python (https://github.com/egraphs-good/egglog-python)
+ python/tests/test_unstable_fn.py:177: error: Unused "type: ignore" comment  [unused-ignore]

Expression (https://github.com/cognitedata/Expression)
- README.py:498: error: Name "z" already defined on line 496  [no-redef]
- README.py:572: error: Name "z" already defined on line 570  [no-redef]
- README.py:628: error: Name "xs" already defined on line 211  [no-redef]
- README.py:632: error: Name "mapper" already defined on line 207  [no-redef]
- README.py:633: error: Name "predicate" already defined on line 208  [no-redef]
- README.py:634: error: Name "folder" already defined on line 209  [no-redef]

AutoSplit (https://github.com/Toufool/AutoSplit)
- src/utils.py:147:12: error: Name "win32ui" is not defined  [name-defined]
- src/utils.py:155:29: error: Name "ctypes" is not defined  [name-defined]
- src/utils.py:156:5: error: Name "ctypes" is not defined  [name-defined]
- src/utils.py:159:9: error: Name "ctypes" is not defined  [name-defined]
- src/utils.py:160:9: error: Name "ctypes" is not defined  [name-defined]
- src/utils.py:163:19: error: Name "win32gui" is not defined  [name-defined]
- src/utils.py:176:20: error: Name "FilterGraph" is not defined  [name-defined]
- src/utils.py:182:12: error: Name "COMError" is not defined  [name-defined]