feature request: add --locked option to depends and prevents

[kwfinken]

When using the composer depends and composer prevents commands, it would be useful to have a --locked option, similar to composer show --locked and composer outdated --locked. Our docker based workflow typically deals with composer using only the composer.json and composer.lock file in our repository, not creating the vendor directory until the system is deployed to docker containers.

[Seldaek]

This probably should use a mechanism similar to (or perhaps we can even reuse it outright) the ShowCommand, which supports loading --locked or installed and if nothing is installed warns.