Deps: Nested dependencies aren't symlinked/downloaded

[obiwac]

If we have a dependency tree (for gv) that looks like this:

kos:/home/obiwac/.cache/bob/deps/kos.ae856b9ae362fccb.local
	proto:/home/obiwac/.cache/bob/deps/proto.8aed25a1d18461de.local
		header:/home/obiwac/.cache/bob/deps/header.facdb7cf42c785c3.local
	header:/home/obiwac/.cache/bob/deps/header.facdb7cf42c785c3.local
	umber:/home/obiwac/.cache/bob/deps/umber.b34dcaeec11a942b.git
proto:/home/obiwac/.cache/bob/deps/proto.8aed25a1d18461de.local
	header:/home/obiwac/.cache/bob/deps/header.facdb7cf42c785c3.local
umber:/home/obiwac/.cache/bob/deps/umber.b34dcaeec11a942b.git

The header (local in this case) dependency never gets symlinked, because only the direct deps of gv are downloaded. We should download everything.