It's often useful to run a module with `python -m`, e.g.:
python -m pyinstrument script.py
Running a python script this way was hard, though, as `spack python` did
not have a similar `-m` option. This PR adds a `-m` option to `spack
python` so that we can do things like this:
spack python -m pyinstrument ./test.py
This makes it easy to write a script that uses a small part of Spack and
then profile it. Previously thee easiest way to do this was to write a
custom Spack command, which is often overkill.
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