Simplify Command boilerplate for typical use-cases

[totten]

Overview

This is a general shuffling the Command class hierarchy. The basic Command from symfony/console is the same, but several of the helpers used by cv commands have been moved around.

Most notably, the BaseCommand (internal to cv-app) has been replaced by the CvCommand (which is now part of cv-lib). The contract for CvCommand should be easier to use, and -- when we get CiviCRM extension integration (#219) -- it should slot-in more easily.

Before

• The BaseCommand class is internal to cv-app. It is fairly heavy with helpers that don't need to be in there (low cohesion).

• Pretty much all commands require the use of BootTrait. The boilerplate code involves importing the trait, activating the trait, and then figuring out where to call a few helper functions. This is done in every command-class.

After

• The CvCommand class is part of cv-lib, and it includes BootTrait.

• Some other helpers have been moved out to standalone utilities.

• Common bootstrap options (--level, --user) are declared in BaseApplication. Bootstrap behavior is automatically included by way of CvCommand. The default behavior assumes that you want Civi to be booted before running the command. But this can be overridden via getBootOptions().

Comments

1. This can be viewed as a code-cleanup on the existing commands.
2. This can be viewed as a contract-break for the command classes. However, there rare mitigating factors: plugins.md referred to this part of the functionality as experimental. Plugin support was only recently merged. So... I don't think there's much of an install-base needing the old contracts of BootTrait.
3. I'm working toward a subsequent PR where cv commands are loaded from civicrm extensions. The thing is that... civicrm extensions are only loaded post-boot. The options like --level and --user should still work, but they can't work the same. (The Command class hasn't been loaded... so we have to start processing those flags before have the Command.)