add `package_path` to non-Fireplace ES API results (bug 1029186)

https://bugzilla.mozilla.org/show_bug.cgi?id=1029186

it's already in the app detail resource:
http://localhost:8000/api/v1/apps/app/marketplace/

this adds it to ES:
http://localhost:8000/api/v1/apps/search/?q=marketplace

r? @robhudson 
I was going to ask about this but thought you knew what you were doing. I'll know better next time.