Add package support to source-repositories

Adds a new "package" type to source-repositories. When the package type
is specified in an origin-repository-* file the package name and type
"package" need to be specified like so:

nova package

The existing map-packages mechanism can be used to map the specified
package name to the actual distribution package names.  The
pre-configured package repositories on the image will be used to install
the packages during the install.d phase of the image build.

If the repo type is package, an environment.d hook script is updated
with an environment variable which is sourced by dib-run-parts. This is
so that other hooks, particularly in install.d, will know what repo type
was used and can make the correct decision about doing a source install
or not.

Change-Id: Ief4e524664cd32d2087aeb3b1766f05e017de91e
Merge "Add package support to source-repositories"