Various improvements to exec (#3894)
Various improvements to exec (#3894)

* Various improvements to exec

- Handle __future__ imports
- Fix errors from invalid references (e.g. via sentry exec bin/load-mocks)
- Dont include configure() handler when already present in code

@getsentry/infrastructure

beeftornado/sentry,BuildingLink/sentry,fotinakis/sentry,beeftornado/sentry,zenefits/sentry,gencer/sentry,BuildingLink/sentry,alexm92/sentry,ifduyue/sentry,JamesMura/sentry,gencer/sentry,mvaled/sentry,fotinakis/sentry,BuildingLink/sentry,ifduyue/sentry,fotinakis/sentry,mvaled/sentry,JamesMura/sentry,ifduyue/sentry,looker/sentry,mvaled/sentry,jean/sentry,gencer/sentry,looker/sentry,zenefits/sentry,jean/sentry,ifduyue/sentry,mvaled/sentry,mvaled/sentry,jean/sentry,looker/sentry,gencer/sentry,zenefits/sentry,JamesMura/sentry,BuildingLink/sentry,JackDanger/sentry,looker/sentry,JackDanger/sentry,JackDanger/sentry,BuildingLink/sentry,gencer/sentry,mvaled/sentry,alexm92/sentry,ifduyue/sentry,jean/sentry,JamesMura/sentry,fotinakis/sentry,zenefits/sentry,beeftornado/sentry,jean/sentry,zenefits/sentry,looker/sentry,alexm92/sentry,JamesMura/sentry