This patch modifies the in-tree build backend to build sdists that swap
out pointers to it in the `pyproject.toml`'s `[build-system]` section.
The effect of this is that the first build from source (for example,
from a Git checkout) uses our PEP 517 in-tree build backend. But the
produced tarball has `build-backend` set to `setuptools.build_meta`
which is the native build backend of `setuptools`. So any following
builds from that sdist will skip using the in-tree build backend,
calling the setuptools' one.
The good news is that if the first build generated the manpages, they
will be included and won't go anywhere even though, a different build
system is in place.
Combined with #80253, this will make sure not to modify the current
source checkout on that first build.
Co-authored-by: Matt Clay <matt@mystile.com>
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