Indicate deprecated ports

[chrstphrchvz]

Similar to how a prominent yellow-box message is displayed obsolete ports, I think it would be nice for a similar message to be displayed for ports that have been marked as deprecated (using the deprecated portgroup), with information similar to as displayed by the port command: details on why it's deprecated (i.e. it is EOL or unmaintained upstream, or because MacPorts has encountered issues maintaining it) and that it could potentially be removed from MacPorts in the future.

(The deprecated portgroup also allows specifying a maximum compatible OS or Xcode version for a port, but I'm not sure those are reasons to display a deprecation message on the port's page, since they do not normally display such a notice during installation on compatible systems.)

[mojca]

The information about the port being obsolete comes straight from PortIndex whenever there's a replaced_by keyword in it.

I checked the obsolete PG, but not even port info tells you that the port is deprecated (I tested with ruby23).

Nearly all the info about ports comes directly from the PortIndex. Occasionally a bit of extra work is done by parsing something like port variants <portname> to extract descriptions of the variants.

We need to have an easy-to-get way of extracting the deprecation info from the port. How can we do that (other than writing a dedicated tcl script)?

@jmroot