Check for cemerick/piggieback and fall back to cider/piggieback

[dpsutton]

CIDER injects cider/piggieback automatically now while some users are
still on cemerick/piggieback. The result is that this will always
return true since cider/piggieback is on the classpath.

Notably, shadow-cljs fakes a cemerick/piggieback and places the
compiler environment there for cider-nrepl. But we see the injected
cider/piggieback and so look for the cljs compiler environment there
rather than in cemerick.

see clojure-emacs/cider#2305

[bbatsov]

I'll merge this, but @thheller should change it soon, as once I switch the middlewares to target the new nREPL server cemerick/piggieback will no longer work at all, and leaving this check would make things confusing, as no one would imagine it's here for shadow-cljs's sake.

[thheller]

Sorry about that. Totally forgot about this. Just pushed [email protected] which should properly support both or either namespace.

[bbatsov]

Thanks for tackling this!