Add search key and pseudo-port selector for port groups

[raimue (Rainer Müller)]

Similar to other field selectors it should be possible to select ports by which port groups they require. This should work both in search

Example Portfile:

PortGroup foo 1.0
PortGroup bar 1.5

name      blah
...

Expected commands and output:

$ port echo portgroup:foo
blah
$ port echo groups:bar
blah
$ port search --groups foo
blah @...
   ...
$ port info --groups blah
groups: foo 1.0, bar 1.5

The field PortInfo(portgroups) already exists. It is just not written into the PortIndex. It is currently a nested list of port groups and the corresponding version: {{foo 1.0} {bar 1.5}}.