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.

TODO:

- [x] Implement
- [x] Mention in how-to docs
- [ ] tests
- [ ] check for other docs to update