vendored apps w sdist: install common build deps in staging

sdist packages have 2 kinds of dependencies: build-time deps and
runtime-deps. Before PEP-517, there was no standard to specify a package's
build-time deps, but with PEP-517, a package defines its build-time deps in
its pyproject.toml.

In an online install, pip reads it and downloads build-time deps and
builds the sdist. In a vendored (offline) install, this is not possible
because "pip download" is not smart enough to download build-time deps
during vendoring (github.com/pypa/pip/issues/8302).

Before pip 23.1, when build-time deps were missing, pip fell back to
using the legacy 'setup.py install' method. pip 23.1 removed this
fallback and started enforcing PEP 517 (https://discuss.python.org/t/announcement-pip-23-1-release/25844).
Therefore, for vendored apps with sdists, we install the 2 most common
build-time dependencies - wheel and setuptools. These are packaged by
the dependency pipeline within the "pip" dependency.

Author: Arjun Sreedharan

fixtures/vendored/pre-pep517-sdist/Procfile

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+web: python3 -m http.server 8080

fixtures/vendored/pre-pep517-sdist/README.md

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+# Vendored sdist app pre-PEP517
+
+This is a vendored app that installs sdist of a package (oss2) that does not
+follow PEP-517.
+
+`pip download` (i.e. vendoring) only downloads the run-time dependencies to the
+output directory, and not build-time dependencies.
+
+Before PEP-517, the python ecosystem did not have a standard to specify
+build-time dependencies of packages. This app installs such a package.
+
+oss2 is only officially supported until python 3.8. See https://pypi.org/project/oss2
+
+Vendoring for this app is done with the following command on a bionic env:
+
+```
+apt-get install -y python3.8 && update-alternatives --install /usr/bin/python3 python3 /usr/bin/python3.8 1 && pip install pip==24.0 && pip download --no-binary=:none: -d vendor -r requirements.txt'
+```

fixtures/vendored/pre-pep517-sdist/requirements.txt

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+oss2

fixtures/vendored/pre-pep517-sdist/runtime.txt

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+python-3.8.x