Verify package name(s) when using an update whitelist

I found out about the `composer update nothing` command/hack at Symfony Live London. This seems like it is occasionally useful, but is only possible as the package name list isn't verified. This means that misspelling a package name would have the same effect, without the user necessarily realising that they've made a mistake.

This verifies the requested name(s) against the list of packages, returning an error when one isn't found

So:

    php composer.phar update monolog/monolo

Would result in:

    Package monolog/monolo not known

The `nothing` hack is still permitted (as long as it's on its own, not in a list of package names).