create-exe does not work with packages with multiple commands

[fschutt]

When generating an executable from a package with multiple commands, we need to create the exe in a way where we can do:

output.exe --command-name command

[fschutt]

When creating the output.exe with create-exe, if the package specifies multiple commands, we should be able to generate the executable even if entrypoint doesn’t exist.

[syrusakbary]

@fschutt let's use --command (or -c in it's short form) instead of --command-name (we should also change that on the CLI in a backward compatible way)