configure: add --enable-pypi and --disable-pypi

In the event that there's no vendored source present and no sufficient
version of $package can be found, we will attempt to connect to PyPI to
install the package if '--disable-pypi' was not passed.

This means that PyPI access is "enabled by default", but there are some
subtleties that make this action occur much less frequently than you
might imagine:

(1) While --enable-pypi is the default, vendored source will always be
    preferred when found, making PyPI a fallback. This should ensure
    that configure-time venv building "just works" for almost everyone
    in almost every circumstance.

(2) Because meson source is, at time of writing, vendored directly into
    qemu.git, PyPI will never be used for sourcing meson.

(3) Because Sphinx is an optional dependency, if docs are set to "auto",
    PyPI will not be used to obtain Sphinx source as a fallback and
    instead docs will be disabled. If PyPI sourcing of sphinx is
    desired, --enable-docs should be passed to force the lookup. I chose
    this as the default behavior to avoid adding new internet lookups to
    a "default" invocation of configure.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-Id: <20230511035435.734312-23-jsnow@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

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