HHVM build-deps: add condition to exec, move to contint

* There's no sense in introducing a whole class for calling a single exec, much
  less introducing two (hhvm::packages and hhvm::packages::dev), especially
  since the HHVM packages are not in the packages scope. This should be done in
  the contint module which needs it.
* Add an onlyif condition to the exec so that it doesn't run on each Puppet
  run, causing log churn.

Change-Id: I884423c48c59a8abe4c0aa78306cde6ffb5e5c58