Raise an error if Igniter is installed as a non-dev dependency

[sevenseacat]

Is your feature request related to a problem? Please describe.

I added Igniter to one of my projects to use it, via adding to the mix.exs file:

{:igniter, "~> 0.5"}

This worked to install Ash, but then errored because Ash installs sourcerer as a dev/test-only dependency, whereas Igniter needs it as a full dependency.

Igniter should not be installed this way, as I learned - it should only ever be a dev dependency (and doesn't even need to be added to your deps to use it if the igniter_new archive is installed).

Describe the solution you'd like

Igniter should raise an error if it is detected that it is installed incorrectly. This should be possible to do!

[zachdaniel]

I've put this into the "Someday" category because all of the current guides and processes make it a dev/test dep, but this is a nice to have in case its not set up properly in a project 😄

[ken-kost]

This should be possible to do!

I tried to figure out how; failed. 😖
Seems to me key information here is the project's env from which the dep is called.
Is that even possible to get from inside the dep igniter? Feels like it isn't. idk 🐛

[zachdaniel]

The there isn't really a good way, thinking about it. Maybe the best thing to do is call something out in the getting started guide. "Make sure that igniter..."