Allow subcommands to be created by namespace packages

[goodmami]

Just like packages in the delphin.codecs namespace become available for the delphin convert command, we could allow namespace packages to register subcommands for the delphin command-line interface. One way of doing this is to crawl all top-level packages and look for some metadata, similar to how the codecs work. But this would slow down command invocation because all modules would need to be imported. If backward compatibility weren't an issue, we could make delphin.commands into a namespace package and just look for modules in that directory, but then existing imports (like from delphin.commands import mkprof; mkprof(...)) would not work as mkprof would need to be a module and modules are not callable. Instead we could make a new namespace like delphin.cli.

It would work like this:

• delphin.main crawls modules in delphin.cli
• if one of these modules contains the appropriate code, a subcommand is created for it

The "appropriate code" could be some metadata that delphin.main uses to construct the subcommand, or some function like register() that takes the top-level argument parser and adds the subcommand itself.