[prebuilt_package] Compute package set for prebuilt packages

This makes it possible to add prebuilt packages to the
available/preinstall set and have the correct indices computed.

Test: fx set with topaz/packages/prod/chromium in --available and
--preinstall and --args=enable_chromium=true

Change-Id: I7ef8b12a5066b72cef98267ac4a6abf5e337e3d6
1 file changed