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Add '-m <module>' to the shell so that a module can be run with long
tracebacks in the way that 'python -m <module>' runs it. This takes the
approach of module-script-loading, but uses runpy (which is what Python
itself uses) to find the module, instead of a copy of the import machinery:

* It works on every supported version of Python. The earlier version
  imported 'imp', which Python 3.12 removed, and used code that is only
  valid for Python 3.
* A package is run through its '__main__', and relative imports within it
  work, as the package, spec and loader of the module are set up.
* sys.argv[0] is the file of the module, as for Python.
* Everything after the module name is for the module, including anything
  which looks like one of our options (so '-m module --flag' keeps the
  flag), and '-mmodule' without a space is accepted.
* The module is found from the current directory, and the modules that it
  imports are found there too, rather than where the module is.
* A module which is not found gives an error and an exit code of 1.

The tests run modules with Python and with the shell, and compare what the
module sees and the exit code that results.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>