updates around RPM packages on CentOS 7.x

updates around RPM packages on CentOS 7.x

Add explicit dependencies on packages that are possible to
remove without removing the `perl' RPM from CentOS 7.x.
However, simplify deps.perl to avoid naming essential Perl
modules (e.g. `perl-Exporter') that cannot be removed
without removing the `perl' RPM entirely.

We can also drop `Socket6' from deps.perl since it's pulled
in as-needed by other packages and we try to avoid it in favor
of the (now-standard) IO::Socket::IP.