Accept packages as arguments about lint

[dsymonds]

I'm not sure we want to try to do this. For instance, gofmt only operates on files and directories; it is "go fmt" that permits package arguments, and that's really the go tool expanding the package arg into a list of files for gofmt to run over.

Folk are stewing over ways of tools to plug in to the go tool in this way; govet is another example. I think that's the right strategy rather than pushing knowledge of packages down into every individual tool.