gh-123011: Fix warn_explicit() with the globals of the __main__ module (GH-155318)

The __main__ module executed as a script or a command has __spec__ set to
None, so warn_explicit(module_globals=globals()) emitted a spurious
DeprecationWarning.  It also raised ImportError when the loader was unable
to provide the source of the module: when the module was executed with -m
(the loader can only handle its own module name) or as a command (the
built-in importer has no source).