Restore 1.2.x timing for config calculation

[dfreeman]

This will fix #166 (or at least the original error reported there). Now, a typo in an import path will no longer give the cryptic error message:

Unable to resolve styles from addon ; is it installed?

Instead, devs will see:

Unable to resolve styles module 'oops/a/typo' imported from
'<importing file>.css'. No virtual module with that name was
defined and no corresponding addon was found.

It also updates the guidance in the virtual modules documentation due to the config calculation timing changes that were necessary to support component/template colocation.

[dfreeman]

This PR should now fix #166 completely—it both addresses the awful error message and restores the timing of options calculation back to the way it works in the current stable release. I'm aiming to get another beta release out either today or tomorrow (depending on how some of the Embroider stuff shakes out) and this will be included in that.