Make it possible to use importmap and propshaft together when generating

[jcoyne]

No description provided.

[jrochkind]

i see you deleted this, but swapping the order of propshaft vs importmaps would have broken the build in my for-discussion #3360 -- but it could have been fixed again with somewhat more complicated logic and/or actual arguments telling the asset generator which path you want.

Let's talk about it, if not in a community meeting that didn't happen this month, maybe on #3360 , or in slack, or on a voice call we set up, or something!

[jcoyne]

The switching the order was something we did in #3340, so that would have rebased out of this PR.

[jrochkind]

Ah, you're right I got confused -- in fact #3360 depends on ImportMap before Propshaft, so indeed the same thing that is in here and #3340.

(If reversed, the logic would just have to be made more complicated ot support #3360).

Thanks!