SG-40807 Fix raise FileNotFoundError

[carlos-villavicencio-adsk]

Root Cause Analysis

This seems to be a non-backwards-compatible regression introduced on #1047. The issue is in import_handler.py's find_spec method (lines 278-296). When a module like QtPrintSupport is requested from sgtk.platform.qt5:

1. os.path.isdir(.../qt5/QtPrintSupport) returns False (no such directory)
2. A SourceFileLoader is created pointing to .../qt5/QtPrintSupport.py — this file doesn't exist, but SourceFileLoader doesn't validate at creation time
3. spec_from_loader() also doesn't validate — no exception is raised
4. find_spec returns a valid-looking spec, claiming it "found" the module
5. Python's import machinery uses the spec's SourceFileLoader to execute the module
6. SourceFileLoader.exec_module() tries to read the non-existent file → FileNotFoundError
   The old imp-based code never had this problem because imp.find_module() would raise ImportError immediately if the file wasn't found.

What happens with the fix:

1. Python calls CoreImportHandler.find_spec('sgtk.platform.qt5.QtPrintSupport', ...)
2. The handler resolves the parent sgtk.platform.qt5 from sys.modules and gets its __path__
3. It computes module_file = .../qt5/QtPrintSupport.py
4. New check: os.path.isfile(module_file) → False → returns None
5. Python tries remaining finders (PathFinder etc.) — none find QtPrintSupport either
6. _find_and_load_unlocked gets spec = None → raises ModuleNotFoundError (subclass of ImportError)
   So the customer's except ImportError: block will correctly catch the error again, just like before the imp → importlib migration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.42%. Comparing base (c7eb43e) to head (e1bc6df).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1088      +/-   ##
==========================================
+ Coverage   79.40%   79.42%   +0.02%     
==========================================
  Files         198      198              
  Lines       20738    20744       +6     
==========================================
+ Hits        16466    16475       +9     
+ Misses       4272     4269       -3     

Flag	Coverage Δ
Linux	78.88% <100.00%> (+0.02%)	⬆️
Python-3.10	79.23% <100.00%> (+0.02%)	⬆️
Python-3.11	79.14% <100.00%> (+0.03%)	⬆️
Python-3.13	79.13% <100.00%> (+0.02%)	⬆️
Python-3.9	79.19% <100.00%> (+0.01%)	⬆️
Windows	78.91% <100.00%> (+<0.01%)	⬆️
macOS	78.89% <100.00%> (+0.02%)	⬆️

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[julien-lang]

LGTM! :)