source_on_disk() so far relied on the 'sourcePackage' field, which contains the
primary dependency of a torcx packge (app-torcx/docker ->
app-emulation/docker). Now the 'metaPackage' field (app-torcx/docker) is used,
which lets us look at RDEPENDS and figure out all packages that are indirectly
installed when installing a torcx package. torcx_dependencies() does just that,
so move it's definition to torcx_manifest.sh.
Signed-off-by: Jeremi Piotrowski <jpiotrowski@microsoft.com>