Allow "." as synonym for current port directory

[ryandesign (Ryan Carsten Schmidt)]

Many shell commands use . to indicate the current directory. I often run commands like git add . or git commit .. It's only natural that from time to time I also end up trying to run port commands with ., such as port lint . but this currently gives an error:

Error: Port . not found

Might we allow . to indicate the current directory? MacPorts already operates on the current directory when no port name is specified, but since git defaults to operating on the entire clone rather than the current directory, I am now in the habit of specifying . on commands.

[jmroot (Joshua Root)]

This gets a bit complicated because we don't currently allow using a bare path to specify a port to operate on, only a port name, pseudo-portname, or full porturl. If we accept ., users might quite reasonably assume that any directory would be accepted in the same place. The -D option is what currently allows you to specify a path to cd to before proceeding, which changes the evaluation of the current pseudo-portname among other things.