Support relocatable rpmbuild by adding data attribute to the toolchain

[reutermj]

rpmbuild ships with runtime configuration files (lib/rpm/) that must be present for it to successfully build RPMs. When running under remote execution, these files were not being staged because the toolchain had no way to declare them as dependencies.

Add a data attribute to rpmbuild_toolchain so that runtime files can be specified alongside the rpmbuild binary. These files are propagated through RpmbuildInfo and included as inputs to the MakeRpm action in pkg_rpm.

Expanded examples/rpm/prebuilt_rpmbuild to demonstrate usage of the API. The existing example uses mocked rpmbuild, and this adds mocked lib/rpm/ runtime data files

[aiuto]

Could you add an example of the use under examples/rpm.
I think it is probably something like a glob of /usr/lib/rpm, but I'm just guessing.

[aiuto]

The code looks good. The example will make it useful to others.

[reutermj]

Thanks @aiuto. Updated the existing prebuilt rpmbuild example to use the new attribute.
Also tested it in with a toolchain module: https://github.com/reutermj/toolchains_rpmbuild_prebuilt

[aiuto]

buildifier complaining. I fixed the line.

[reutermj]

Thanks @aiuto!