Don't show source-only packages in package list

Don't show source-only packages in package list

external-source: packages which have no binary package will be marked skip:
and so won't show up in setup.ini at all.

If we change to using the Source: line to identify a source package, which
is (by definition) source only, these will appear in setup.ini, but without
any install: lines, so we need to explcitly avoid showing them in the
package list.

Future work: perhaps add another view to show just source packages?