"port upgrade" error message usability

[Andrew Janke]

Hi, MacPorts developers,

Long-time Homebrew user and recent MacPorts convert here.

Minor usability issue with the `port` program, I think: I suspect that a 
common operation for regular MacPorts users to do is "upgrade all my 
stuff to the latest version".

I tried doing this with `port selfupdate`; `port upgrade`, and got this 
error message:

[~] $ sudo port selfupdate
--->  Updating MacPorts base sources using rsync
MacPorts base version 2.7.1 installed,
MacPorts base version 2.7.1 downloaded.
--->  Updating the ports tree
--->  MacPorts base is already the latest version

The ports tree has been updated. To upgrade your installed ports, you 
should run
   port upgrade outdated
[~] $ sudo port upgrade
Can't map the URL 'file://.' to a port description file ("Could not find 
Portfile in /Users/janke").
Please verify that the directory and portfile syntax are correct.
To use the current port, you must be in a port's directory.
[~] $

I'm a dev with 25 years of coding and sysadmin experience, and I don't 
know what to do with that error message. I dunno what a regular user is 
supposed to do with that. (Yes, I saw the "To upgrade your installed 
ports" output from the selfupdate command, but still.)

Maybe the error message here could be modified to include a "maybe you 
meant `port upgrade outdated`" message or something like that? Where's 
the 'file://'" coming from, anyway? Does `port upgrade` operate on some 
port definition found in the current working directory by default? I did 
not provide a URL as an input to the `port upgrade` command, so it's a 
little unexpected that I got an error complaining about a URL here.

Cheers,
Andrew