Rename or extract the solidus executable

[elia]

Using solidus as the executable name makes little sense for a gem called solidus_dev_support, we should establish a base solidus command within core and allow it to be extended by the ecosystem.

solidus_dev_support can keep using the extension namespace but should deprecate defining the executable.

A possible alternative could be to switch to solidus-extension with commands such as:

• generate for new extensions
• update similar to rails app:update
• release

[aldesantis]

Worth adding that we also discussed with @elia the possibility of moving the solidus executable to solidus_core and simply adding the extension subcommand in solidus_dev_support. This seems the cleanest route, since it gives us the ability to have commands such as solidus extension generate and solidus extension update that make a lot of sense semantically.

[elia]

@aldesantis 💯

The tricky part about that is supporting older versions, so maybe we could resurrect solidus_cmd for that purpose, moving it to thor and leaving out the extension generation… then reintegrating it in side solidus_core with the next release.