Fix package count for openSUSE. 3.7

[mutageneral]

For some reason, for the package count for OS "opensuse" is set to "packages_opensuse", but that function does not exist, resulting in the error ./disfetch: 262: packages_opensuse: not found and no package count info on the fetch.

The existing functions that are compatible are "packages_rpm" and "packages_zypper". I used hyperfine to benchmark both, and using rpm directly is faster than using zypper.

This simple pull request sets the package count grabbing method of OS "opensuse" to "packages_rpm".

[mutageneral]

Note, you could also remove the zypper package count retrieval method line, as my benchmarking shows it is useless, because rpm is faster and will always be available when zypper is. Tell me if you want me to add that.

[mutageneral]

Oops, I just realized all distro that use packages_rpm are on a single line and I modified the opensuse line instead of moving opensuse to the line for packages_rpm.

[q60]

much thanks for your contribution