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://launchpad.net/bugs/1845157

LP: #1845157
Closes: #939790