runner/autopkgtest: when finding src pkg, skip binary pkgs for other archs

When looking for the src pkg, the entire Package-List is searched for every binary package name. However, there are some rare source packages that provide binary packages for specific arch(s), while the same binary package is provided by another source packge for a different arch. One example is the 'binutils' and 'binutils-mipsen' source packages; currently the 'binutils-mipsen' package provides several 'binutils-mips*' named binary packages for intel archs, while the main 'binutils' source package provides the same-named binary packages specifically for the mips arch.

In cases such as this, autopkgtest needs to make sure to only consider source packages that provide the binary for the current testbed arch, otherwise it will be confused about what source package name and what version to look for in the archive.

An example of this is: https://bugs.launchpad.net/ubuntu/+source/autopkgtest/+bug/1845157

(LP: #1845157)

Closes: #939790