Expand the set of commands that a package script can implement

[catamorphism]

In a pull request I'm about to submit, I'm changing the interface with package scripts so that they only can implement custom logic for the "install" command. Package scripts should be able to reimplement any rustpkg command, so allow for that.

[steveklabnik]

Is this actually fixed with pkg.rs?

[amaranth]

No, pkg.rs only lets you work on the "install" command.

[alexcrichton]

rustpkg has been removed