Don't issue `no-redef` with conditional imports

Bug Report

When an exception is caught from a block with a single import statement, one can be certain that the import failed (right?).

When an import fails, a to-be-imported function is not defined.

If a function is not defined, mypy should not issue a no-redef message.

To Reproduce

pip install overrides

"""Feature request: detect failed imports and suppress no-redef."""
try:
    from overrides import overrides
except ModuleNotFoundError:
    # mypy issues no-redef
    def overrides() -> None:
        pass

print(overrides)

Expected Behavior

No message

Actual Behavior

bug.py:5: error: Name "overrides" already defined (possibly by an import) [no-redef]
Found 1 error in 1 file (checked 1 source file)

Your Environment

• Mypy version used: mypy 0.982 (compiled: yes)
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.10.8

Riproduci lo snippet dell’issue con mypy 0.982 su Python 3.10.8, usando la configurazione predefinita e senza flag della riga di comando. Traccia come vengono analizzati l’import condizionale e la definizione di fallback; il lavoro è completato quando questo caso non segnala più no-redef senza introdurre regressioni nei controlli esistenti.