gh-87465: Bypass sys.modules when finding module specs in pyclbr - #161864

pyclbr._readmodule() previously called importlib.util._find_spec_from_path(), which checks sys.modules before searching search_path. Because pyclbr maintains its own _modules cache and inspects source files from the search path without importing them, consulting sys.modules caused two problems:

1. Browsing any module with import __main__ (such as pdb or doctest) when sys.modules['__main__'].__spec__ is None (or another synthetic module in sys.modules without __spec__) raised ValueError instead of treating an unresolvable import as ModuleNotFoundError.
2. When __main__.py existed on the given search path, sys.modules['__main__'] shadowed it with the running process's own __main__ module.

Switch pyclbr._readmodule() to call importlib._bootstrap._find_spec(fullmodule, search_path) directly, remove the temporary_main_spec() workaround from Lib/test/test_pyclbr.py, and add regression tests.

• Issue: pyclbr.readmodule_ex traversing "import __main__": dies with ValueError: __main__.__spec__ is None / is not set #87465