Make 'raco pkg install' with no arguments install the current directory.

This is useful for telling people how to install a new pkg, from
GitHub or elsewhere: just get the files, and then do
`raco pkg install` in the relevant directory.

Also, both cabal (the Haskell package manager) and npm (the node.js
package manager) behave this way.

To explicitly get the old behavior, specify the sources as
`--pkgs pkg-srcs ...`.  This is useful in scripts, when `pkg-srcs`
might be empty.