Allowing PortGroups to bump port revision

[Jeremy Huddleston Sequoia]

In https://trac.macports.org/ticket/54744, we've been pondering adding a PortGroup to allow concurrent installation of multiple providers of the OpenSSL API (eg: openssl, libressl, libressl-devel) and allow ports to specify which they are compatible with.

It occurred to me that it would be nice if we could update the PortGroup when one of the dependencies changed their dylib id rather than revbumping all ports.  Is this something anyone else has considered?

--Jeremy