python/wheels: add vendored meson package

In preference to vendoring meson source, vendor a built distributable
("bdist" in python parlance). This has some benefits:

(1) We can get rid of a git submodule,
(2) Installing built meson into a venv doesn't require any extra
    dependencies (the python "wheel" package, chiefly.)
(3) We don't treat meson any differently than we would any other python
    package (we install it, end of story, done.)
(4) All future tarball *and* developer checkouts will function offline;
    No git or PyPI connection needed to fetch meson.

Note that because mkvenv prefers vendored packages to PyPI, as mkvenv is
currently written we will never consult PyPI for meson. (Do keep in mind
that your distribution's meson will be preferred above the vendored
version, though.)

```
jsnow@scv ~/s/q/python (python-configure-venv)> python3 scripts/vendor.py
pip download --dest /home/jsnow/src/qemu/python/wheels --require-hashes -r /tmp/tmpvo5qav7i
Collecting meson==0.61.5
  Using cached meson-0.61.5-py3-none-any.whl (862 kB)
Saved ./wheels/meson-0.61.5-py3-none-any.whl
Successfully downloaded meson
```

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-17-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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