'Requires' dependencies are not built

[euanh]

We build a package's 'BuildRequires' dependencies before we rebuild the package, but we never build a 'Requires' dependency. This affects Python packages, where a package might have a runtime dependency which is not a build dependency because there is no build step.

[euanh]

For example, linux-guest-loader requires xcp-python-libs at runtime, but not a build time. xcp-python-libs will still be built because it is a dependency of the 'all 'target, but some other packages buildrequire linux-guest-loader, and since there is no build dependency between linux-guest-loader and xcp-python-libs, xcp-python-libs might not have been built at the time linux-guest-loader is installed, causing the installation to fail and stop the build.