Check existence of required runtime deps at beginning of commands that need them

[stackotter]

At the moment if someone tries to build an app on Linux, they'll get some error about failing to move a dynamic library. The underlying cause (which should at least be more visible with the new error system) is that they haven't installed patchelf, the tool that Swift Bundler uses to update rpaths.

Eventually we will ideally either ship all required dependencies with Swift Bundler, or reimplement required features within Swift Bundler itself if they're 'simple' enough. But for now we should warn users up-front that they're missing required runtime dependencies.

Ideally we should only warn the user when the tool is actually going to be invoked somewhere in the processing of the command that they've run.