Ignore packages created by functional tests
Store intermediate packaging products in var/cache

Using world-writeable directories (like /tmp) can cause symlink attack
vulnerabilities.

* Only Packagers::MacPkg uses this for now. AIX packaging still uses
  /tmp
* The cache dir can be relocated by configuration, so an additional
  option for `package_tmp` is added to the config.
* Add delegation in the Project class so that packagers use _only_ the
  project for input.