osc build needs to learn the 2-phase approach of OBS with %generate_buildrequires

[DimStar77]

Feature Request
RPM has, for a while now, known the section %generate_buildrequires, which can run any script/tool and extract BuildDependencies by whatever means it deems fit for purpose.

OBS itself supports this and runs a 2-phase progress on the spec file:

• Build the spec file, up to the %generate_buildrequires section
• Extract the output .src.rpm which contains the generated dependencies, and restart the build using the build-dep information provided by this.

It would be nice to build such packages locally - and not only by OBS.

There are a few test packages in https://build.opensuse.org/project/show/home:dimstar:meson

Describe the solution you'd like
OBS implemented support for this in openSUSE/open-build-service@eb087cf (and a few follow ups)

It would be great for osc to learn how to deal with that too